Is there any update on the "cannot select INBOX, is this a valid transport?". My installation also stopt fetching mails on the 27-december. I have tried a couple of things (removed and added the email adresses in osTicket an removed and added the registration in the microsoft tenant), but nothing seems to have any effect. I can send email with the Microsoft 365 account, but i cant read the emails from the microsof365 accounts.
I can't disable ipv6 on the server because there a more sites on the same server witch need ipv6. For now i have a little workaround to forward the mails to a external mailbox witch a can read with IMAP.