I’ve just experienced the same problem (internal software error:.\Vobulator\TitlePlanner\CGOPGrouper.cpp,line 401) My project consists of 9 short videos where the viewer can navigate through alternate scenarios for each of the 9 videos. There are between 6 to 11 chapter marks for each time line, no overides, no looping menus and no clips shorter than 2 seconds. The problem only occured after I had made some changes for the client. I had replaced some clips which resulted in gaps in some of the timelines. Rebuilding the timelines to remove the gaps solved the problem.
Keith