Same here. There seems to be a global problem after some change or update. So far no solution yet.

Thanks for the response Thomas. Hopefully the problem can be resolved without further delay or i will have to migrate to another server.

i

If you are using ESET/NOD32 security/AV then probability is very high it is causing your problems. According to their forums, you should temporarily disable SSL until they solve the problem per https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products?ref=esf - the problem is being reported by dozens of users https://forum.eset.com/topic/26517-problems-with-thunderbird-and-imaps/ https://forum.eset.com/discover/

The 'unsupported protocol' error is caused by a conflict between the TLS on the mail server and the version required by TB:

https://support.mozilla.org/en-US/questions/1295861

See https://support.mozilla.org/en-US/kb/thunderbird-78-faq#w_after-upgrading-to-thunderbird-78-i-cannot-receive-or-send-email-messages