try this:
start any q4max demo (which works). then when the demo is running, just try to run this demo.
if you all did this automatic (q4replay) and it still doesnt work, try to do all the above manually and make sure demo_enforceFS 1 is on.
That's the way it worked with me, because even with demo_enf.. 1 I got the classname error (playing it with q4replay).