If it is a subscription thing, you can fix it by:
- Finding the user's .sub file, usually under /var/imap/user/j/joe.sub or something similar.
- Shut down cyrus
- Edit the .sub file and delete the offending line
- Start cyrus
- Get the user to re-subscribe to the folder from Thunderbird
I don't have any .sub files or directories named that way in my cyrus folders....
They'll be somewhere. Use locate or find to find out where.
As root: find / -name "*.sub"
It may take a few minutes, but that should find them.
