kmail doesn't start anymore

so, well, I seem to have messed up my setup somehow, kmail won't start 
anymore.
I get the error message attached, which seems to complain about a 
collection which cannot be created.
in akonadiconsole, however, I can see all relevant agents are running, 
emails (IMAP) are there.
how can I solve this without deleting all kmail/akonadi ressources and 
configs ?

_______________________________________________
KDE PIM users mailing list
Subscription management: https://mail.kde.org/mailman/listinfo/kdepim-users