[Exim] closed connection after STARTTLS errors

Suddenly I am starting to get this message a lot and it's causing
massive problems, what can cause it?

We have two mail servers here, the new exim server and the "legacy"
CommuniGate server. Users who are still configured to use IMAP on
CommuniGate have an alias forwarding their mail to the old domain.
Unfortunately, whenever exim tries to forward a message, it gets
the error "closed connection after STARTTLS errors" and the message
is frozen.

If I force the message with exim -M, it gives the same error message
but it is delivered. Any idea what could be causing this problem? I
tried commenting the tls* lines out of /usr/exim/configure, but the
problem persists. I'm starting to dig around in the exim source.

Thanks,
Mark