Thanks for the reply.
Because of the unknown errors while rendering longer project in Premiere as well as the long time to open and closing these projects, I split tem up into smaller ones. I also find in to be an advantage, if I find out that I have to do minor change and to not have to render the whole project again. But the main reasons is the unknown error while render......
As consequence, I have more files (DD 5.1) behind each other on the timeline.
I also tried to work with multiple timelines. But then, I cannot navigate within the finished blu ray from one timeline back to the first without going via the menu which is no solution for 'normal' users.