Thunderbird creates a folder called INBOX on the IMAP server, and also a Trash folder inside it.
I guess it happened when I modified the settings, to move deleted messages to the Trash folder (on the IMAP server). There is already in a folder called Trash on the server.
When I choose
Account > Server Settings > When I delete a message: Move it to this folder I see my IMAP root as
Inbox. I can choose any of the existing folders. But something somewhere goes wrong, and instead of using the
Trash folder on my IMAP root, TB creates a folder named INBOX and tries to access folders inside it.