I had a script that would update the refresh and access token if the request wasn't successful. It worked updating the refresh / access token throughout the day yesterday. I tried running the script this morning and I was given the error:

{"error":"invalid_grant","error_description":"Refresh token has expired"}

I read that refresh tokens were not supposed to expire for 60 days. Is there a bug somewhere or do refresh tokens not remain valid for extended periods of time?