Yep, me too. I wasn’t able to log into the Teams desktop app for about 2 days because of it. I finally got tired of having to use the browser for this, so I did a bit of sleuthing. I ended up at this tech community post with a successful fix posted by @Arayn_Raje.

The fix for me was to:

1. Sign out of Teams; if that is not possible then at least fully quit the app using the icon in the System Tray or exit out of it using the Task Manager.
2. Go to the Teams shortcut on the file system; for me it is at %USERPROFILE%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs.
3. Right-click the icon and select Properties.
4. On the dialog that opens, click the Compatibility tab.
5. Check the box inside the Compatibility mode section on that tab and make sure the selection is set to Windows 8 (it should be by default).
6. Click Apply.
7. Open Teams again — it should work now; sign out and sign back in to make sure the change isn’t a fluke.
8. In Teams, check for updates; there should be one.
9. VERY IMPORTANT: Be sure to sign out of any other Teams accounts other than the one that just successfully signed in.
10. The Compatibility mode setting can be removed now.

So, what is going on here? Well, it seems that Microsoft has removed the capability to be simultaneously logged into both work and personal Teams accounts. I can’t definitively say this, but the following statement — visible when you try to use Team’s personal account switcher to get to your other accounts — seems to corroborate my findings:

If you attempt the Sign out here without also signing out of Teams via the desktop application, you WILL get the same error that started this whole mess. << sigh >>