All I can offer is to - delete account - exit and restart thunderbird - set up account but OMIT the password until account is setup - enter password only when first attempting to retrieve mail.

I was trying to set up an IMAP connection.

The default options on the create account screen don't let you add account without password. Won't autodetect 'stuff'

Change it to POP3 and it creates instantly.

Checking that the zone records are set up correctly on the domain. Added complication, nameserver is with a different host to domain and email account. Oh, what a tangled web we weave!

Actually, setting IMAP works fine without password. I've given that advice to many with never a problem. If problems persist, please post account settings and will try to assist.