The page you mentioned has "The controls on this page are disabled." at the top. Just for grins I tried to save the changes after I checked the Enable Categories box and it returned the message "This function is not active in the Trial Version."
So....it looks like the option has been disabled. Thanks for pointing me to a part of the CP that I hadn't seen yet. Now I know that the problem is not on the server but just a disabled option.
Onwards to more adventure........
