Comment 8 for bug 1735402

How did you determine that this environment is "fully functional"?

If the problem is indeed with this credential, you would not know that it has expired until you do something that requires it like upgrade-juju or add-machine or similar...

So if you cannot upgrade juju or add a machine, you'd need to:
(1) get a new credential (they expire after some period of time, say a year... you'd know best when this one is),
(2) update credential's copy on the controller using update-credential command (we do not have means at the moment to see what credential controller has unless you want to examine database. Most of CRUD credential commands look at your local copy. 'update-credential' is the ONLY one that interacts with the controller's copy... It's the gap we wanted to fill for a while);
(3) then you should be able to do your desired operation - upgrade-juju, etc...