The symptoms: You are editing a show in show builder and items will not drop in the right place, when you drop an item it moves to a different time location.
The cause: This seems to be caused (strangely) by the Default Crossfade Duration in Preferences. It seems that if you have a positive value ie. 15.0 in there it will disrupt the showbuilder so I would advise setting it to zero while you use showbuilder and then setting it back afterwards.