We're having this problem for a long time without being able to know the cause...
Sometimes when we try to login the access fails despite in the session log says that the authentication it's ok
QuoteSuccessful login for drubio (ID: 22) from 195.71.223.33 in session 79b8jat...
To solve this, we have to drop the cookies for the site, and then we can log in again...
On the error log, when this happen we can see:
Quote[25-Feb-2015 07:56:40 +0100]: DB Error: could not find driver in /rcube_explotacio/program/lib/Roundcube/rcube_db.php on line 158 (POST /index.php?_task=&_action=login)
Some clues??
At least some info on what kind of error is this "DB Error: could not find driver??"
It is what is says, the database driver could not be found. Check what type of database are you using. Check that you have the PDO driver for that installed on your server. For example if you are connecting to a MySQL database then you need pdo_mysql.
You could also try enabling the installer, that has a check section which I think will test that you have the database drivers you need installed - along with other bits Roundcube needs.