Teams Error Code - 80080300 on Login

I had the same issue. Exactly same. Here's what you need to do -

1. Sign out, if possible or at least close the app
2. Go to properties (Right-click on the icon)
3. Compatibility Tab
4. Run compatibility troubleshooter and complete it
OR
4. Directly set Compatibility Mode to 'Windows 8 '
5. Open the Teams app again

In the end, this is what I have done, you can just uncheck the run with compatibility mode and try closing and opening the app. If it works normally, well and good. If it doesn't, repeat the steps.

I'd also suggest that if your app now is working properly, go to the 'Settings and more' and select 'Check for updates'. Teams will update it, in the end, you just need to 'Refresh now'.

Hope this helps. It surely did for me.