I recently upgraded from 0.8.1 to 0.8.4, and now, in the rc error log I can see:
[17-Jan-2013 08:37:59] MDB2 Error: not found (-4): no RDBMS driver specified
[17-Jan-2013 08:37:59 +0100]: DB Error: no RDBMS driver specified in /aplicacions/webmail/roundcube-0.8/program/include/rcube_mdb2.php on line 107 (POST /rcube/?_task=login&_action=login)
I ran the upgrade script from the 0.8.4 folder, setting as target my running 0.8.1 folder...
Roundcube is running fine (or at least it seems to me), but this is a bit suspicious...