You can get this error when your browser experiences caching conflicts.
To fix this, here are recommended steps:
- Use Incognito window. File > New Incognito Window
- Sign out of other Google accounts you might be signed into
- Clear cache and cookies
- Use Chrome and set up one Chrome profile per email address. Instructions: https://support.google.com/chrome/answer/2364824?co=GENIE.Platform%3DDesktop&hl=en
- Keep your browser up to date