If you are using #archlinux, think twice before updgrading (mariadb). Upgrading to v. 12.1.2 will break #akonadi, and thereby #kmail, #kontact, #korganizer etc.
The cause of this is not kmail or akonadi but mariadb. If you experience this, you can downgrade mariadb packages using pacman -U and point it to packages in /var/cache/pacman/pkg/ - the packages mariadb, mariadb-libs and mariadb-clients must be downgraded, adding them on one line is best.
UPDATE I have NOT got this working safely yet.
If you were hit
* close any kdepim app - kmail, kontact, korganizer, kaddressbook, merkuro or other
* stop akonadi - $ akonadictl stop
* remove ALL akonadi related dat from ~/.local and ~/.config. ALL.
* I removed all config, state and app data for kdepim apps as well, before kmail would start cleanly
Repeat until kdepim apps starts cleanly
