Comment 10 for bug 1747646

Perhaps the OAuth token is expiring during long backup jobs and needs to be renewed? Not sure what it is based on because our times vary widely.

https://developers.google.com/drive/api/v3/handle-errors#401_invalid_credentials

This happens to me after first-time backup running for about 45 minutes. Then it fails with (same as OP's log):
Giving up after 5 attempts. Error: gdata-service-error-quark: Authentication required: {
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "authError",
    "message": "Invalid Credentials",
    "locationType": "header",
    "location": "Authorization"
   }
  ],
  "code": 401,
  "message": "Invalid Credentials"
 }
}

Ubuntu 18.04.01 using:
deja-dup 37.1-2fakesync1
duplicity 0.7.17-0ubuntu1