I'm also having the exact same issue. I did find that the script completes successfully if I uncheck "Generate drizzle data" under Image Registration, however, I lose the drizzle data (obviously). I can see from the console that the FITS keyword that is used by the script is WBPPWGHT, so I suppose I could just use the script to calibrate and register, then integrate manually using this keyword, but it would be nice if the script could integrate using the drizzle data. Hopefully this bug gets fixed soon.