I have the same problem in latest SVN. Using ffdshow, but have unchecked xvid support. Black screen when running full screen, normal. When I use 14:9 for example everything works ok or not using mediaportla in fullscreen.
Can someone upload a short (nonpirated) clip that is giving this behavior. If it is only on xvids, than most likely one of the experimental builds was used for encoding. If this is the case, we can't do much about it. If anyone know (exactly) which build broke this, I'll take a look at the problem.
I can confirm this issue is still present in the current SVN. It's only with certain versions of the XVID codec that this happens. I am not using ffdshow and the file plays fine in WMP and VCL outside of MediaPortal.