Comment 14 for bug 1429725

A little update: I could not manage to get a verbose output from APT, only a complaint about update-alternatives not finding /etc/mysql/my.cnf.fallback which I did not think to be relevant.

However, this gave me an idea: from a backup I've restored the /etc/mysql config from a MySQL installed before the MariaDB migration. This solved my problem. Now how and why my /etc/mysql directory became corrupted will - I think - remain a mystery...

So the conclusion is: always back up your /etc :-)