I've had this error before, but I could get it to stop if I messed around for a bit. But today I can't get it to go away, even after restarting. Much more complex sequences have been rendered before. This particular section has a background still, a repeating video clip and three still images above it with luma keys on them. At first the 3 stills were jpegs, so I reconnected them to pngs, but that didn't help.
The Mac Pro I'm using has a 2.66 Quad-Core Xeon and 8GB of 1066 DDR3.