Have you checked the "Access Protection Log" Weather this option is preventing the user to do certain modification.
type this command in run %deflogdir% and verify the access protection log.
If you already checked access protection logs why cont you try to change the security by disabling the access protection option instead of disabling the McAfee.its just try
Yes I did check and nothing about this, not even an entry for the current day (I did try several time during the day so it should have been in there).
It does work if I disable only the access protection.
So, I know it's the access protection but I have nothing in the logs about it.
Any idea about the policy that blocks this?
If its work after disable access protection
then kindly check this option weather it set to block if it so then you can untick the block option.
Access Protection Properties->Anti-Virus Standard Protection-Prevent User Rights Policies from Being Altered