For some reason Maria Database's default open file setting does not seem to work well. Even with moderate load we get too many open files error which leads to mysqld problems which leads to dovecot auth problems etc. Quite a lot of secondary effects. What could be opening so many files on DB that won't close?
I recommend that you just raise the limit. I heard about that error message several times recently, most likely MariaDB changed a default value and the new value is too low fro production systems.
