Signature verification failed

I’m testing out JWT on a couple of my sites. Both sites return a token as expected when using postman.

My app can connect to one of the sites without any issues and returns a valid token. My other site returns:
{
"code": "jwt_auth_invalid_token",
"message": "Signature verification failed",
"data": {
"status": 403
}
}

Both sites are using Version 1.3.4.
What are my options to debug this issue? The only thing I’m doing is changing the url for the JWT endpoint.