Check to see if Thunderbird is set up in the Firewall as an allowed program. It is possible the update is seen as a completely different program - confused Firewall.

Exit Thunderbird. Access Firewall Set Thunderbird as allowed program If already set as allowed, then block it or remove and ok everything. Then access Firewall again and reset Thunderbird as allowed. Start Thunderbird.

Firewall isn't on, every other download has worked without issue. Thanks though.

It's in an endless circle jerk saying "checking mail server capabilities" if that provides any clues...