Error fetching folders: syntax: expecting 's', found 'c'

Error fetching folders: syntax: expecting 's', found 'c'

Both yesterday and today, I have been unable to fetch email using my mail client. All I get is this error message:

Error fetching folders: syntax: expecting 's', found 'c'

I have no idea what it means, but it sounds like the mail server is not sending the folder information correctly any more or something. How do I fix that?