Also ... this might be relevant.
When I go to FOLDER MANAGEMENT, I see Inbox and sshd listed as folders.
Inbox is checked as SUBSCRIBED but sshd is not.
I don't know why sshd would be listed as a folder for all users.
Also, when I click the plus sign to add a folder, the choices for PARENT FOLDER are --- and sshd.
The --- selection is the default.
I've tried creating a folder with either choice and get the same error.