Invalid_grant error happening occasionally for a given access token

Hello Everyone,

For one user we've seen that the access token was sometimes returning a invalid_grant, but ahead and later requests with exactly the same token did succeed.

This happened on sugarondemand.com hosted instance.  Below I include response headers:

"Strict-Transport-Security": "max-age=31536000",
"Content-Length": "81",
"Connection": "close",
"Status": "401 Unauthorized",
"X-Application-Server": "ondemandus....",
"Cache-Control": "no-store",
"Expires": "Thu, 19 Nov 1981 08:52:00 GMT",
"X-XSS-Protection": "1; mode=block",
"Pragma": "no-cache"

The response body was  { "error":"invalid_grant", "error_message":"Your authentication token is invalid." }.

We use our platform name to request access credentials.

What can be a reason of such quite peculiar random 401 responses?
Any help will be appreciated, thanks.