Following https://userbase.kde.org/Kmail/Configuring_Kmail/Accounts/Office_365, I successfully created an EWS resource.

I was able to subscribe to certain mailbox folder, hence it received to folder list successfully, but not any mails are fetched and akonadi shows:

org.kde.pim.akonadiserver: Error while handling command FetchCollections on connection akonadi_ews_resource_2 (0x55b872a25050)
Please register the custom scheme 'urn' via QWebEngineUrlScheme::registerScheme() before installing the custom scheme handler.
org.kde.pim.ews.client: Starting OAuth2 authentication
org.kde.pim.ews.client: Launching browser for authentication
org.kde.pim.ews.client: PKeyAuth certificates not found
js: Uncaught TypeError: n.saveSharedData is not a function
js: Uncaught TypeError: n.saveSharedData is not a function
js: Uncaught TypeError: n.saveSharedData is not a function
js: Uncaught TypeError: n.saveSharedData is not a function
org.kde.pim.ews.client: Authentication succeeded
setMap: "access-token"
setMap: "refresh-token"
Please register the custom scheme 'urn' via QWebEngineUrlScheme::registerScheme() before installing the custom scheme handler.
org.kde.pim.akonadiserver: Error while handling command FetchCollections on connection akonadi_ews_resource_2 (0x55b872a25050)

Also, the authentication dialog is displayed on every akonadi/kmail start-up, 
it's not able to remember the login data (well, this is okta), and the web page requests akonadi permissions every time.


Operating System: openSUSE Tumbleweed 20210524
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.82.0
Qt Version: 5.15.2
Kernel Version: 5.12.6-3-preempt
OS Type: 64-bit
Graphics Platform: X11
Processors: 24 × AMD Ryzen 9 3900X 12-Core Processor
Memory: 62.7 GiB of RAM
Graphics Processor: GeForce GTX 760/PCIe/SSE2

Kmail: 5.17.1
kdepim-runtime: 21.04.1

All TW "official" builds, no special repos in this respect.