Okay, there's the error log. I didn't see it in my FTP client.
The errors file ends on 28 Jan 2011. There are no current entries in it. When I want to log in, it takes a few seconds and then simply says that authentication failed and I'm back at the login form. No more information.
PHP 5.3 on Ubuntu Linux, fully compatible with Roundcube 0.5.1 but not so much with 0.5.3.