SUMMARY
Since the last update (roughly early December 2020) starting Kmail triggers a new browser window pointing to https://accounts.google.com/o/oauth2/auth/identifier... [credentials omitted for privacy] as if it's unable to store the password to my gmail account. This was tested with both a personal and a business account.

STEPS TO REPRODUCE
1. Have Kmail installed and configured to access a Gmail IMAP account
2. Launch Kmail 

OBSERVED RESULT
A new window of the default browser is triggered as soon as Kmail starts, asking for authentication

EXPECTED RESULT
Seamless authentication happening behind the scenes followed by instant access to the inbox

SOFTWARE/OS VERSIONS

Operating System: KDE neon 5.20
KDE Plasma Version: 5.20.4
KDE Frameworks Version: 5.77.0
Qt Version: 5.15.2
Kernel Version: 5.4.0-58-generic
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-6200U CPU @ 2.30GHz
Memory: 7.1 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics 520

ADDITIONAL INFORMATION
Konsole output running Kmail:

Connecting to deprecated signal QDBusConnectionInterface::serviceOwnerChanged(QString,QString,QString)
org.kde.pim.akonadiserver: Starting up the Akonadi Server...
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
Connection to speech-dispatcher failed
akonadi.collectionattributetable                   OK
akonadi.collectionmimetyperelation                 OK
akonadi.collectionpimitemrelation                  OK
akonadi.collectiontable                            OK
akonadi.flagtable                                  OK
akonadi.mimetypetable                              OK
akonadi.parttable                                  OK
akonadi.parttypetable                              OK
akonadi.pimitemflagrelation                        OK
akonadi.pimitemtable                               OK
akonadi.pimitemtagrelation                         OK
akonadi.relationtable                              OK
akonadi.relationtypetable                          OK
akonadi.resourcetable                              OK
akonadi.schemaversiontable                         OK
akonadi.tagattributetable                          OK
akonadi.tagremoteidresourcerelationtable           OK
akonadi.tagtable                                   OK
akonadi.tagtypetable                               OK
org.kde.pim.akonadiserver: Running DB initializer
org.kde.pim.akonadiserver: DB initializer done
Connecting to deprecated signal QDBusConnectionInterface::serviceOwnerChanged(QString,QString,QString)
org.kde.pim.akonadicontrol: Reference to unknown agent type "akonadi_notes_agent" in agentsrc, creating a fake entry.
org.kde.pim.akonadicontrol: Akonadi server is now operational.
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0003f20) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d001ae20) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d001b140) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0024cb0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d002e5b0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0037e20) )
Cannot initialize model with data QJsonObject() . missing: QJsonValue(string, "urls")
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00414b0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d004a910) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0053c80) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d005cfc0) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0053c80) identified as "AgentBaseChangeRecorder - 94444912592912"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00791f0) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00791f0) identified as "AgentBaseChangeRecorder - 94794252682128"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00824a0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00952a0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00955b0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d009e870) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00a7c80) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d001ae20) identified as "AgentBaseChangeRecorder - 94008920818624"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0003f20) identified as "AgentBaseChangeRecorder - 94519557703280"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00cb2b0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00d47e0) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d002e5b0) identified as "AgentBaseChangeRecorder - 94226514368592"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d001b140) identified as "AgentBaseChangeRecorder - 93917078981552"
qt.qpa.wayland: Wayland does not support QWindow::requestActivate()
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d004a910) identified as "AgentBaseChangeRecorder - 93994635635744"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00414b0) identified as "AgentBaseChangeRecorder - 94876669517568"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d01267d0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0126610) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d012f4b0) )
org.kde.pim.akonadiserver: Subscriber "" disconnected
org.kde.pim.akonadiserver: Subscriber "" disconnected
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d00a7c80) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0141de0) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0037e20) identified as "AgentBaseChangeRecorder - 94074278859248"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00cb2b0) identified as "akonadi_maildispatcher_agent - 94074278845360"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00d47e0) identified as "SpecialCollectionsMonitor - 94074278954096"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00a7c80) identified as "MessageViewerMonitor - 94792124050288"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00a7c80) identified as "MessageViewerMonitor - 94792124050288"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0141de0) identified as "TagActionManagerMonitor - 94792128034304"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0141de0) identified as "TagActionManagerMonitor - 94792128034304"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0024cb0) identified as "AgentBaseChangeRecorder - 94761366112784"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0143f00) )
org.kde.pim.akonadiserver: Subscriber "" disconnected
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0144b20) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0144b20) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0186b90) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d005cfc0) identified as "AgentBaseChangeRecorder - 94025752795472"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0144b20) identified as "Archive Mail Kernel ETM - 94025752876000"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0186b90) identified as "ArchiveMailCollectionMonitor - 94025752893040"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d01a9310) )
org.kde.pim.akonadiserver: Subscriber "" disconnected
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d01b23c0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d01b23c0) )
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d01bf520) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00952a0) identified as "AgentBaseChangeRecorder - 94453809021808"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d01a9310) identified as "MailFilter Kernel ETM - 94453809100192"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d01b23c0) identified as "MailFilterCollectionMonitor - 94453809129024"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d01bf520) identified as "MailFilterItemMonitor - 94453809100288"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d01267d0) identified as "KMail Kernel ETM - 94792123352368"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0126610) identified as "SpecialCollectionsMonitor - 94792123991648"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d012f4b0) identified as "MessageListTagMonitor - 94792123918368"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d0200240) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00824a0) identified as "AgentBaseChangeRecorder - 94489414137120"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d00955b0) identified as "AgentBaseChangeRecorder - 94667400808992"
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0200240) identified as "UnifiedMailboxChangeRecorder - 140734576824000"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d02232a0) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d02232a0) identified as "SpecialCollectionsMonitor - 94453809607744"
org.kde.kgapi: Unauthorized. Access token has expired or is invalid.
org.kde.pim.google: Got error: KGAPI2::TaskListFetchJob(0x562f599d50c0) "Autenticazione non valida."
org.kde.kgapi: Unauthorized. Access token has expired or is invalid.
org.kde.pim.google: Got error: KGAPI2::CalendarFetchJob(0x562f599b49f0) "Autenticazione non valida."
org.kde.kgapi: Unauthorized. Access token has expired or is invalid.
org.kde.pim.google: Got error: KGAPI2::ContactsGroupFetchJob(0x562f599d5160) "Autenticazione non valida."
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d0143f00) identified as "FilterManagerTagMonitor - 94792127776368"
org.kde.pim.akonadiserver: New notification connection (registered as Akonadi::Server::NotificationSubscriber(0x7fd1d023ef80) )
org.kde.pim.akonadiserver: Subscriber Akonadi::Server::NotificationSubscriber(0x7fd1d023ef80) identified as "MessageListTagCacheMonitor - 94792136448400"
org.kde.pim.akonadiserver.search: Executing search "kmail2-3492219703-SearchSession"
org.kde.pim.akonadiserver.search: Search  "kmail2-3492219703-SearchSession" done (without remote search)
org.kde.pim.akonadiserver: Error while handling command DeleteItems on connection akonadi_google_resource_0 (0x55d89fd6ff10)
org.kde.pim.akonadicore: Deleting items from the akonadi database failed: "No items found"
org.kde.pim.akonadiserver.search: Executing search "searchUpdate-1609146367"
org.kde.pim.akonadiserver.search: Search  "searchUpdate-1609146367" done (without remote search)
org.kde.pim.akonadiserver.search: Search update for collection "DeclinedInvitations" ( 25 ) finished: all results:  0 , removed results: 0
org.kde.pim.akonadiserver.search: Executing search "searchUpdate-1609146367"
org.kde.pim.akonadiserver.search: Search  "searchUpdate-1609146367" done (without remote search)
org.kde.pim.akonadiserver.search: Search update for collection "OpenInvitations" ( 24 ) finished: all results:  0 , removed results: 0