[Dovecot] 'append aborted'

I'm copying all my archives from one imap server on one machine to the imap-ssl dovecot server on another machine.

After a bit, there's an error that's tossed up on my mozilla client that says the imap server couldn't complete the operation with the message append aborted.

It keeps popping up after anything from 50 to 300 emails.

Last night I was able to move ~12000 emails from one machine to the other without the ssl enabled. Any hints on what to look for?