I don't think e-on can have carried all the fixes in 4.12 over to the RenderCow engine. I can now, finally, render the rain tutorial OK as a single frame, but the rain doesn't appear if I animate the scene and use my render farm. The fact that somebody gets a "flashing" effect is interesting - sounds as if some machines on the network are redering correctly, others not. Machine specific issue? Or maybe different versions of RenderCow on different machines?