just found some info about this error in the P7 forum at content paradise. It has to do with using depth mapped shadows (it won't happen if you use raytraced shadows) and with rendering in a seperate process.

Teyon over at CP wrote on March 11:

"We're already aware of it and working on a fix but thanks! We expect this to be corrected fairly soon.
Meant to add that if you turn off Separate Process, the problem most likely will correct itself. That's a temporary work around until the fix is ready."