As I stated above, JCE does not use the string "Restricted Access!" anywhere in it's code, so this must be cuased by some other process on the site, therefore it is not a JCE issue.
Uninstalling an re-installing JCE Pro (to clear any possible legacy code) does not solve the issue.
Unfortunately I am unable to suggest any other possible solution here. As you say that this issue is onyl on this one site and not others, confirms that the cause of the issue is specific to this site, and therefore must be caused by an installed extension.
Ryan Demmer
Lead Developer / CEO / CTO
Just because you're not paranoid doesn't mean everybody isn't out to get you.