It works again now, weird.
Actually I may have an idea of the reason. There was some problems with my e-mail arrangements (I have to read my IMAP over SSH tunnel), for the SSH connection spewed out some errors to stdout once in a while, but the connection itself was fine. I took care of that and now it works.
This should probably be marked as a bug in KOrn, as in such a failure it shouldn't freak out, but at most show an error message.
I'm posting this here, if someone might find this useful and because I don't think it's possible to delete threads (the epitome of embarrassements for asking questions before checking things more thoroughly...)