Just go to
portal.office.com. I have to go there because it´s a school account,
login.live.com deosn´t work for that.
which NM version you're using?
BTW NM27 works as well.
Problem's already been resolved, it's an issue relating to the hell known as user-agent sniffing... given how meaningless user agent strings have got in the last six years or so I'm still uncertain why websites feel the need to do feature detection via it.
The Teams issue might be related to WebRTC, though. Pale Moon has it removed because the developers happen to dislike WebRTC, over any other actually feasible reason. Serpent or Yandex.Browser are effectively your only choices, though you might need to spoof as Google Chrome with both to avoid discriminatory behaviour.
(Meanwhile, I split the thread from XP Luna's post onwards. I felt like it didn't concern the RT1 installer but was worthy of its own topic, explaining why.)