Problem and Solution
If you open SyncWith and it is a blank gray screen, or you get an error
drive.google.com refused to connect, it’s probably because you’re signed in to multiple Google accounts. To fix the issue, close SyncWith, sign out of your other Google accounts, use incognito browser, or sign in to your Google account in a different browser, then open SyncWith again.
This is unfortunately caused by a bug in Google's Workspace Addon system, see
If you're using Chrome, please try the Profiles feature which will give you separate browser sessions for each account, avoiding these types of problems.