Are you saving the flight plans using the CDU (.rt format)? Or saving them in the .PLN format using the default FSX Flight Planner, or a third party flight planner?

I don't save the flight plan (.rt format) with the SID and STAR included. While the SID is normally fine, I always have to redo the STAR so that the landing RWY is added to the flight plan, and the ILS Freq/Hdg is included in the NAV RAD page of the CDU.

Edit. If you have the Captain Sim 777, the .rt files are not compatible with the Captain Sim 757 & 767 CDU, which might be the cause of your 757 crashing when loading a plan.