akonadi needs mariadb (not newer mySql) to start up. Other things might be involved too. Since akonadi gets used by address book, the notes, kmail, PIM, etc. any problem there will bleed over.
Hopefully some of the info there will fix your specific issue.
This seems to be related to this bug: https:/ /bugs.launchpad .net/ubuntu/ +source/ akonadi/ +bug/1445383
akonadi needs mariadb (not newer mySql) to start up. Other things might be involved too. Since akonadi gets used by address book, the notes, kmail, PIM, etc. any problem there will bleed over.
Hopefully some of the info there will fix your specific issue.