I had to help someone else with this problem, and took the time to figure out where the file is on Windows.
If other attempts to sign in are still failing, close SketchUp and delete login_session.dat for your SketchUp version. As an example, the login_session.dat file for 2021 on Mac is in this folder:
~/Library/Application Support/SketchUp 2021
You can get to that folder by using Go to Folder… on the Go menu in Finder, and paste in the text I gave.
For Windows it’s in:
%AppData%\SketchUp\SketchUp 2021\SketchUp
Again, you can get there by pasting the text I gave into the path field in File Explorer.
Reopen SketchUp and sign in should work.