Authorize Google Ads
This captures a fresh refresh token that matches your current OAuth client - the real fix for an invalid_grant error.
Step 1. Open Google's consent screen and approve access with the account that owns your Manager (MCC):
Step 2. After you click Allow, your browser tries to open http://localhost and shows a "can't reach this page" error - that is expected. Look at the address bar: it now contains ...?code=SOMETHING&scope=.... Copy that whole address (or just the code) and paste it below:
Note: your OAuth consent screen must be set to "In production" in Google Cloud Console. In "Testing" mode Google expires the refresh token after 7 days, which brings back invalid_grant.