I have recently got a Plesk 7.5 for windows. I created some domains (e.g. somedomain.com) in Plesk and when I try to connect to the webmail of a domain (webmail.somedomain.com) I got the following error:
Appliance Error (internal_error)
An unrecoverable error was encountered: ""
This problem is unexpected. Please use the contact information below to obtain assistance.
For assistance, contact your network support team.
I have using horde as webmail, after some test I found that I can login trough webmail.somedomain.com/horde/imp/login.php but login from webmail.somedomain.com raised the above error. Also I found that when I try to login from an ISP which uses dynamic proxies I get the error and when I connect from a machine with static valid IP everything is OK and I can login to webmail without any problem.
It seems when you are connecting with dynamic proxies, horde enters to an endless loop of redirecting between /horde/index.php and /horde/imp/login.php.
To resolve this general error you should turn off the checkip flag in config.php
Find $conf['auth']['checkip'] = true; and switch it to false
Accually there is two $conf['auth']['checkip'] = true; in config.php and you should switch to false both of them (I really do not understand why there is two same flags for checkip in horde config. It takes near one week to realize that!)
After above changes I restarted the server to ensure that they applied, maybe it was only necessary to restart Plesk.
Thanks to Plesk support team which kindly helps me resolving this error!