That would happen if you select a Google account that's different from the one you used to originally create the Google Doc. This has nothing to do with the email address you used for your Zotero account.

Restart your browser and try again, and choose the correct account.

This is a 403 Forbidden error from Google, and I'm afraid there's nothing we can do about it. If possible, you can try with a VPN to see if connecting from a different IP address makes a difference.

You can also try in a private window just to make sure there are no cookies for any accounts other than the one that created the document.

It's certainly not a global problem and Zotero is working fine with Google docs for people. The 403 HTTP error is known as an authorization error. Google decides to return it for your Google Docs requests, but doesn't include more information, so it's hard to say what the actual cause is.