Resolved -
Microsoft seems to have rolled back the change, we'll be monitoring and working on an implementation that works around this new header, should it roll out eventually.
Mar 6, 09:38 CET
Identified -
We have identified an incident where a subset of users will see error messages when trying to login with their Microsoft account. Upon investigation we discovered that Microsoft seems to be rolling out a set of changed browser headers which are causing issues. We are working on a fix - in the meantime, re-trying to login for a few times seems to solve the issue for most users.
Mar 4, 21:12 CET