[Dovecot] nested folders

[Jim Pazarena]

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!