On Kmail startup I see only "The Akonadi personal information management service is not operational". 
By clicking "Details..." I got a lot of "passed" tests and several failed (short log inside "Additional information".

It happens several times. Sometimes system reboot helps, but now - it doesn't.
The only solution is completely remove ~/.local/{share,config}/akonadi and reconfigure kmail




Reproducible: Always

Steps to Reproduce:
1. Run KMail

Actual Results:  
Kmail not working. Access to emails completely lost.

Expected Results:  
Kmail working.

Test 10:  ERROR
--------

Akonadi control process not registered at D-Bus.
Details: The Akonadi control process is not registered at D-Bus which typically means it was not started or encountered a fatal error during startup.

Test 11:  ERROR
--------

Akonadi server process not registered at D-Bus.
Details: The Akonadi server process is not registered at D-Bus which typically means it was not started or encountered a fatal error during startup.

Test 15:  ERROR
--------

No resource agents found.
Details: No resource agents have been found, Akonadi is not usable without at least one. This usually means that no resource agents are installed or that there is a setup problem. The following paths have been searched: '/usr/share/akonadi/agents'. The XDG_DATA_DIRS environment variable is set to '/usr/share:/usr/share/kde-plasma:/usr/local/share/:/usr/share/'; make sure this includes all paths where Akonadi agents are installed.

Directory listing of '/usr/share/akonadi/agents':
akonadinepomukfeederagent.desktop
akonotesresource.desktop
archivemailagent.desktop
birthdaysresource.desktop
contactsresource.desktop
davgroupwareresource.desktop
facebookresource.desktop
googlecalendarresource.desktop
googlecontactsresource.desktop
icaldirresource.desktop
icalresource.desktop
imapresource.desktop
invitationsagent.desktop
kabcresource.desktop
kalarmdirresource.desktop
kalarmresource.desktop
kcalresource.desktop
kdeaccountsresource.desktop
knutresource.desktop
kolabproxyresource.desktop
localbookmarksresource.desktop
maildirresource.desktop
maildispatcheragent.desktop
mailfilteragent.desktop
mboxresource.desktop
microblog.desktop
mixedmaildirresource.desktop
mtdummyresource.desktop
nepomuktagresource.desktop
newmailnotifieragent.desktop
nntpresource.desktop
notesresource.desktop
openxchangeresource.desktop
pop3resource.desktop
sendlateragent.desktop
vcarddirresource.desktop
vcardresource.desktop

Environment variable XDG_DATA_DIRS is set to '/usr/share:/usr/share/kde-plasma:/usr/local/share/:/usr/share/'


Akonadi itself started:
ps -A | grep akona
 2508 ?        00:00:00 akonadi_control
 2510 ?        00:00:00 akonadiserver