ETNyx
Happend to me as well while trying to add an account which has another subdomain than my mailserver.
I could resolve it by changing the process of obtaining the credentials to “legacy”. You can do that somewhere in the Thunderbird Options (The symbol with 3 horizontal bars on top right and than “Tools” i guess in English).
Cant post a screenshot right now, just from some youtube Video. Maybe someone else can help out. Otherwise i will add it later to this post.
