That's a Quicktime error, but that is the full extent of what we can tell you since you didn't provide any info beyond that. You're not telling us what version of AE yopu use on what system nor do we know anything about your compositions, render settings, fiels used and god knows what else could be relevant for diagnosing this error.
Hey thanks! Im using AE CS6, Trapcode Suite 12.1, Knoll Light Factory 3.0, Elements VC, Compsize 1920x1080 32Bit Float, Workin with Mac Mountain Lion, 3,4 Ghz i7, 16GB 1333Mhz DDR3. Ive got 3 comps inside, with lot of patricles and 3D Elements, wich twisting around and play with each other!