The problem has been around since P5 came out, and the fix is to delete all lights (manually or with a python script) before loading a new light set. The problem is with swapping lights with the same name, but different parameters, and some unexpected code syntax that P5 encounters. SR4 did address most of the occurrences, but not the ones that are due to some conflicts within the lightset file itself such as a conflict between the naming of the light and the naming of the corresponding shadowcam. The reason this is so hard to track is that the freeze does not occur at the time the lightset is loaded. It only occurs when a render is attempted and Poser encounters the conflict which it cannot resolve resulting in an endless loop (not really frozen).