The error message is just the standard "An internal error has occurred. This is most probably a bug, relevant information can be found in the logs. Please look for the details in the /var/log/zentyal/zentyal.log file and take a minute to submit a bug report so we can fix the issue as soon as possible."
I examined the logs in zentyal-usercorner, and zentyal-usercorner/zentyal.log shows this:
2011/11/29 17:58:30 ERROR> ExternalAccounts.pm:259 EBox::Mail::Model::ExternalAccounts::row - Unknown mail protocol: xxxxxxxx
where "xxxxxxxx" is the password for one of the user's external pop-accounts, so it seems to be a parsing-error. ('m guessing that the migration script does not convert the data for external accounts to the new format that the new usercorner expects.)
(Attaching all 4 logs in log/zentyal-usercorner, edited to remove password revealed in log. This is immediately after the migration, installing usercorner and trying to access external e-mail page for one user.)