Hello Richard Baugh,
What authentications types are you using in IIS and what are defined in the web.config
This error could be raised in a scenario like the following:
1. You are logging with a user via Forms Authentication and the login form.
2. The authentication cookie expires after some time of inactivity in the Administrative panel.
3. Following any link in the Admin panel forces login with a user via Windows Integrated Authentication. This user is in a role that does not have an access to the Administrative panel (CMS access
permission), and the error is raised.
You could try granting CMS access
to role Everyone and see whether you will replicate the same problem or not.
Please let us know about the results.
the Telerik team