Here is the actual error. Any ideas?

It started with just one user and now I cannot even log in via the web client. I have rebooted the server.

An unknown application error has occurred. Please correct any errors and retry. If the problem persists, please contact your System Administrator. (service.PARSE_ERROR)

I'm at a loss here. Where would I even start looking for this issue?

Its the weblient only. All other functions are normal. Clients are getting POP3 and IMAP mail just fine.