Hey guys,
I'm having this same problem, but setting $rcmail_config['default_host'] = 'localhost'; did not help.
I have RoundCube 0.8.1 installed on Apache2 with ISPConfig. I've set it up to be a global alias for "/webmail". I have 3 domains resolving to the same server, pwndhost.com, brianhillphotography.ca, itechnews.ca.
My server is "admin.pwndhost.com". I have an email account setup on "brian@brianhillphotography.ca". Whenever I try to login to RoundCube with "brian@brianhillphotography.ca" it just says "Login Failed" below.
Here's what the Mail.log file says:
Sep 18 22:24:42 admin dovecot: imap-login: Login: user=<brian@brianhillphotography.ca>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=2943, secured
Sep 18 22:24:42 admin dovecot: imap(brian@brianhillphotography.ca): Disconnected: Logged out bytes=29/405