[Dovecot] Problem moving folders with Thunderbird

[Loek]

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