I'm having the same problem.
I installed roundcube 0.5.2 on a debian lenny (apache2 + mysql).
Everything went fine during installation precess, also the last tests (smtp and imap).
Now I'm trying to login to round cube and it always says
[04-May-2011 08:23:02 +0200]: IMAP Error: Login failed for mario@aironegifts.com from 192.168.1.249. AUTHENTICATE PLAIN: Authentication failed. in /var/www/roundcube/program/include/rcube_imap.php on line 192 (POST /roundcube/?_task=login&_action=login)
What I find realy strange is that 'from 192.168.1.249' is not my mailserver ip!
In main.php.ini, I have
$rcmail_config['default_host'] = 'mailserver';
and the host name correspond to 192.168.1.248 ip address, NOT 192.168.1.249!
ping mailserver
PING mailserver (192.168.1.248) 56(84) bytes of data.
I tried to change $rcmail_config['default_host'] value with a differnt ip, like 192.168.2.77 but the log message still shows 192.168.1.249.
Roundcube is running on a virtual machine. The phisical server has the ip 192.168.1.249 but there shouldn't be any relation.
grep -R 192.168.1.249 /var/www/roundcube
shows nothing (exept the log), so this ip is not present in any roundcube configuration file.
Finaly, I installed phpmyadmin and searched '192.168.1.249' along the whole database; the only matches were found in the 'session' table.