[Dovecot] Problem moving folders with Thunderbird

I can move around folders to sub-folders in an IMAP account on my dovecot server, but when I try to move a sub-folder back to the top-level (by dragging it to the account name), I get an error:

"The current command did not succeed. The mailserver responded:
Mailbox exists."

It doesn't exist on the server though, and I can create a folder with that name on the top-level. But it is a little annoying. I am using TB 0.7.2. I tried different IMAP server settings in Thunderbird under the account options, but without success.

Loek