[Dovecot] dovecot 1.2.11/ thunderbird 3.1 - moving folders

[Andre Hübner]

Hello List,

customers telled me problem when moving folders in thunderbird which i can confirm. Following setup on my side:
dovecot 1.2.11 with mbox.
Thunderbird 3.1 with ready mailaccount and "Server supports folders that contain sub-folders and messages" is disabled.

i create folder1 for subfolders and in folder1 a folder2 for mails.
Now i create folder3 on toplevel like just like folder1 for subfolders. Now i drag/dop folder2 to folder3.
Thunderbird is moving this folder successful but in folder1/.imap i can still see a folder folder2 which is containing a dovecot.index.log
At this point thunderbird shows error-message when i start to delete folder1. It tells:
[CANNOT] Mailbox is'nt selectable: folder1.
AND
[NONEXISTENT] Directory folder1 is'nt empty, can't delete it.

What to do now?
Is error on my or thunderbirds side?
Parts of my config needed?

Thanks,
Andre