[Dovecot] Unable to delete folder in public namespace

I use public namespace for group mailboxes. Users are able to create subfolders , but its impossible to delete them. thunderbird 2.0.0.21 complains: "Cant' rename mailbox to another storage type", both namespaces (private and public) use same storage type, so I believe that it's a namespace problem.

Is there any way to move folders between namespaces? Or maybe sieve script or plugin which intercept move command and do the move to some unsibscribible folder in public namespace.