Okay, it seems the problem is the cache. I enabled a higher-level debugging option and it showed me the following information:
_doQuery: [Error message: Could not execute statement] [Last executed query: PREPARE mdb2_statement_mysql_4711ddc877b2740db40f0f9ca83067ddaddbf9921 FROM 'DELETE FROM cache\n WHERE user_id=?\n AND cache_key=?'] [Native code: 1267] [Native message: Illegal mix of collations (ascii_general_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE) for operation '=']
I turned off the cache, and it worked. However the mailserver is external as I'm running roundcube from my home PC, so I'd rather enable the cache.
I tried messing around with the collations, but nothing seemed to work. So I've "re-initialized" the database.
Any ideas on what can be done to solve this?