An older project did not work in CS5 due to missing plugins.
I removed all of them and that helped a lot.
Just turning them of is often no option. They keep fouling up the project, no matter how deep they are hidden.
I did get horrible artifacts at some point in a comp; looked like half filled video buffers and alpha masks flashing on and off.
The patterns changed when I changed preview quality. The bit mayhem also could be found in final renders!
After one day experimenting, I found it: CC Toner!
Do not use CC Toner in AE CS5 if unexplainable artifacts appear!!!
CC Toner is only causing strange artifacts, when other plugins are missing.
AE CS4 could work when plugins are missing but AE CS5 can't: It will say "UNEXPECTED DATA TYPE"
It's allmost impossible to remove the missing plugins due to this error message; this should be fixed!