[Cheesehead]

I did some pretty intense testing on this today.
It played through the film once with no errors and on the second playthrough the error occurred about 50 mins in.
It was a section of about 5 mins that wouldn't play without the error.
If I went to the beginning of the film it was fine, end was fine just this one area gave the dreaded 20021.
Frank's fix worked if you overwrote all automation, but not ideal.
I tried various other things and discovered that at least on my session, if I disabled and then re-enabled all my individual DX tracks (not the buses) it cleared the problem.
Can someone else try and see if that works on your sessions and then maybe we can narrow it down to what type of automation causes it?