I have had this issue personally and with others through out my company.
My only insight is it has something to do with our Zscaler proxy app. If Teams shuts down inconsistently it gets stuck in some authentication loop and refuses to pass auth.
I have purged cache, removed all previous folders from Teams. Re-installed to no success.
At the sign in screen attempted to change accounts - no success.
I cannot get fiddler trace to work as Zscaler app will not let this run and taking down the Zapp temporarily fixes the issue. All I see is 407 authentication requirements in the Zapp captures and little to no success on why Team is not passing authentication
properly through the proxy.
We have had all endpoints updates and when it work it seems ok (outside the fact that CPU and Memory consumption is out of control with Teams and the app needs re-written from the ground up).
Open Microsoft case has gone nowhere for 3 weeks now.. Maybe a little less time on fancy chat pop ups and more time on client stability. and oh Yes, Zscaler support does not see an issue either.
so as new clients dog food out in the environments, I can say, the current flavor is not very tasty.