I have had a couple customers in the past few months encounter errors when sending emails using GreenTools for Microsoft (G4MS).

The error that happened showed up in the job log:

"The token has an invalid signature."

The actual JSON that is returned is:

{
	"error": {
		"code": "InvalidSignature",
		"message": "The token has an invalid signature.",
		"innerError": {
			"requestId": "6f42f83c-f09e-4c45-845b-5342c25d39db",
			"date": "2019-03-21T20:42:14"
		}
	}
}

After rerunning the G4MSREGSVC command, the error continued to occur.

How we solved this problem was the user when into their Office 365 account, updated their password, and then re-ran the G4MSREGSVC command.  After that, things worked fine again.

This is just one instance where the error didn't really tell us what was wrong, but having encounted odd errors like this in the past and resetting an account password solved the problem, so we tried it here and things were fine again.

So, when you run into an issue using G4MS or any of it's addons, especially if you haven't changed your password in over 6 months, first change your password then re-register the account using G4MSREGSVC.