[Dovecot] nested folders

using both thunderbird and roundcube with dovecot IMAP, I can create all the folders I desire, but cannot create subfolders; I also cannot delete a folder once created.

I get a very un-informative error message from each. Is there a special setting required to permit sub-folder creation? folder deletion?

I've got 770 mail:mail virtual_user_home 770 mail:mail mail_sub-directory 770 mail:mail .imap 660 mail:mail .subscriptions 700 mail:mail Drafts 700 mail:mail INBOX 700 mail:mail Junk 700 mail:mail Sent 700 mail:mail Trash

Suggestions would be appreciated. Thanks!