invalid_grant error - not the first time

There's been a few reports of this, none with any real answer. Using the iMIS SSO, the process works for "a short period of time" and then starts returning the invalid_grant error message. No changes to the client id/secret, no changes to the underlying code, other than a long weekend there's nothing in between "it worked" and "it failed". Changing the secret seems to get things working again but it fails shortly thereafter.

I really need someone to have a look at this, it's not a matter of copy/pasting the token for testing, it's not an invalid original id/secret. It just eventually stops working.