I had the same issue and this link really helped me. https://www.linkedin.com/pulse/servicenow-outbound-web-services-oauth-20-how-handle-r%C3%A9da-osseili/

The error that SN returned by default is very misleading since it hides the actual detailed error message. We need to add some logs in the script include (OAuthUtil) that parse the get token response, so we can understand the exact issue. 

My particular issue was that I was using the wrong client secret. I used the id instead of the actual secret value. Silly me, I know!

Once you enable the log in OAuthUtil, The error message was able to lead me to resolve my issue. 

Hope this will help someone in the future.