Requesting help for invalid grant mystery

Hello,

We've been running our code for months. We refresh our token every 6 hours automatically with

Once in an while, something like once a week, we will get a return code of 400 with the error message 

'invalid grant: the refresh token is invalid or expired'.   Then we have to go in and do the Auth by hand to get things working again. 

I've set up logging to make sure we are passing in the token that we previously had received. 

Is it possible for you to look at your logs (I have the time for when it first failed in the last episode), and do you have any suggestions on what I might look for?

Thank you