Dovecot mailbox does not allow for inferior mailboxes
Solution 1:
The solution might depend on how you configured the mail_location property.
For instance if you use the mbox storage option, here is an official solution:
# Trick mbox configuration which allows a mail folder which contains both
# messages and sub-folders
mail_location = mbox:~/mail:LAYOUT=maildir++:INDEX=~/mail/index:CONTROL=~/mail/control
But if you are using maildir instead, the documentation seems to assume sub-folders ("inferior mailboxes") are allowed. But somebody else worked around the issue :
mail_location = maildir:~/Maildir:LAYOUT=fs