I don’t have a direct answer. I have run into the same problem trying to register a few master lights from different sessions. The only suggestion I have is to try another approach and see if it works. I suggest the Sirilic script (buried away on the web site). You just dump your lights and calibration files in for color or mono and it automates the stacking process. You could try doing half the files at a time to see if maybe it’s just one hanging you up.