When trying to create folder Files2 on the same level as Files I got an error:
PHP Error: Storage error. Unable to create the folder (error 500)
in IMAP:
[31-Jan-2017 17:55:14,896430 +0000]: [C601] C: A0006 CREATE /Files2 [31-Jan-2017 17:55:14,897636 +0000]: [C601] S: A0006 NO Invalid mailbox name