Open up VTS_01_0.IFO with IFOEdit. Check PGC_1. Make sure it starts with chapter 1. If it doesn't, go to VTS_PGCITI, double click and you should get VTS_PGC_1 below it. Double click on VTS_PGC_1. Now scroll down to the PGC program map. Double click on PROGRAM_1: Entry cell number. Change the value to 1 and save. I hope that's your problem.