Hi Bob, thanks for getting back,
Decided to try the simplest of those first, and in fact manually deleting the core/cache/ folder and then clearing browser cache and cookies (or loading up via an Incognito window) cleared the problem.
Thanks!
Adam
It's always nice when there's an easy fix. Thanks for reporting back.
I had this error too. Was getting 404 errors on various resources throwing this error from the admin CP:
{"success":false,"message":"Access denied.","total":0,"data":[],"object":{"code":401}}
We use CloudFlare to support our site so clearing the cache in that fixed the problem