I'm running MAX/MSP demo on win XP SP2, and it seems to have a bug opening the plugconfig. More than one time out of two, when I first open a patch containing plugconfig, MAX would say such an object doesn't exist.
The workaround I had was to open MAX, create a new patcher, create a plugconfig object and all was fine. Then I coul open my patch. Now it doesn't work anymore... Will try again !