This...
Open finder and go to the following folder: ~/Library/Application Support/Google
- Delete the "Drive" folder there
- Move the Backup & Sync app to the trash
- Empty the trash folder
- Download and Reinstall the Backup & Sync app
- Note that at this point I went through the steps to setup the app from the beginning, but got the same error.
- Support asked me to restart my MBA - this was my first full system restart.
- After restart I went through the setup steps again and everything with the app started working fine. It did a merge to avoid duplicating files, but everything is working fine now.
Points 5 & 6 are key. After reinstalling, the error stills occurs, BUT full reboot worked. On restart, Google login window appeared immediately and files are now downloading.
Hopefully, that's a wrap!