IMHO: The error looks like the "self signed" part is the problem. You probably need to store the public certificate on xs2mail.com, in order the server can validate it. Do you use the _same_ certificate with Dovecot as with UW-Imap? This should give you the same situation as before.
Not the same cert, but the Dovecot one was generated in the same way as the UW-imap one.
So, I'd recommend to try the _same_ one. Did you've storred the old certificate on xs2mail?
You could be right here. I've re-created the certificate in the domain that xs2mail sees but it still gives me the same error. So there must be something about the cert that it doesn't like. Perhaps it does need my public key. It's just a free web service, though - there's no way to do that. Don't know why it worked on the old one, either!
Regards, Phill