wardfixesit

Here is how I did it.
Go to Post SMTP Setup / Show All Settings
Then copy the Client Secret and paste somewhere so you don’t lose it.
Then delete the Client Secret and click Save Changes.

Then Paste the Client Secret back in and click Save Changes.

Now it will ask you to grant permission with Google again, do that and then go back to Post SMTP Setup and send a test email to verify it works.