I've been having a problem lately when I try to render a sequence with an alpha channel in pI (not pI renderer). after the render finishes I get an image sequence but the alpha is only on one side of the image and it's corrupt as well. Sometimes I can actually see the white alpha on top of parts of my image sequence! Any idea on what's causing this?
Sorry I didn't reply sooner. Does this happen with every project, or only specific ones? Try different stage sizes too.
It may be motion blur settings, or video driver issues. Also make sure that you leave particleIllusion in the foreground -- don't move another window in front of it (some video drivers don't handle that well).
I'd suggest trying software rendering, but I know that some combinations of motion blur and alpha channel may cause problems with software rendering.