outlook.live.com: Can suddenly no longer VPN-send from mail client (Thunderbird) on laptop (Win10)

I'm trying to find the cause of this recent malfunction - everything was fine, until days ago; apparently the problem began around the time I changed VPN providers. So only outlook.live.com VPN-sending is affected (mail client, not webmail); without VPN, and receiving are alright; and yahoo.com is 100 % ok.

I would assume it must have something to do with the settings in my Outlook account. Reading and trying to understand all the official instructions (lots of technical terms, and in a foreign language for me) is difficult, I may have overlooked a condition to be met in the settings, or may have not received (inadvertently deleted) a message to confirm that it's me - I don't know. To my impression, the process has become so complicated for a layman like me (with only a certain degree of practical routine) that it just looks like an equation with too many unknowns to be solved.

Would someone see a way out?