Easy: Don't use IE to debug websites.
Internet Explorer has a "feature" that replaces some error messages that webservers send back with its own entirely useless page. The most recent AOLServers attempt to work around this, but I haven't tested them personally to see if it always works. There is also an option on one of the advanced configuration menus to turn off "Friendly" error messages, but again I can't swear that will always fix the problem. Try it, if you still get useless internal server error messages switch to netscape.
Basically the ACS is sending you back an error, probably some sort of access denied message, and you're not seeing it because of IE.
