Are you running the i.s.s.w security product? If so, disabling it or changing its
admin_global plugin execution order to 65535 may fix it.
If you are not running that product, please try disabling all other mods that run on the
admin_global plugin location, then check if it's working. If it is, re-enable them one by one until you find the culprit. Once you've found the culprit, set its plugin execution order to 65535 and it should work
Fillip