Folks,
Today I noticed, that during flight my marked flightplan suddenly turned into a dashed line in map mode. My AP lost the route completely and I had to land the plane by hand.
Why does the flightplan vanish into a dashed line and how can this be prevented?
It would help to let the community know which aircraft this was and which NAV package.
Sounds like a MANSEQ which means AP coupled to the FMS will not track it, you have to handfly the leg.
Or enable HDG and fly the heading 
It happened in the F-35.
I use the GNT750 for crating a flightplan.
As the problem occured I started the flight airborne close to the STAR to Hongkong airport. The plane lost the flightplan (line turned into a dashed one) entering the STAR.
Seems like you have a discontinuity gap in the flight plan. Usually when selecting a SID or STAR from an existing flight plan, there can be a gap at the end of a SID or at the start of a STAR which is disconnected from the main flight path. When this happens, the autopilot nav system will disengage and switched to Heading Mode, in which case the line becomes dashed.
You have to do final checking of your entire flight plan every time you set or change SIDs and STARs. If you see any discontinuity, you need to clear that gap and make sure the waypoint to waypoint path is one continuous line. This way, the autopilot will keep tracking the flight plan throughout the whole flight.
I would add, if you just want a seamless flight and don’t care about following closer to real world procedures then this would be acceptable.
If you are looking to achieve realism, you would fly these vectors as intended. Of course this is probably a shortfall of in-sim ATC as I don’t believe it handles vectors.
If you fly on VATSIM or any other network, do not delete these. Unless you want a controller to get spicy with you
“You’re at the end of my air space and my patience” was a great quote out of Boston the other night
Most of the time I find that you need to manually activate the aproach leg in your flight instrument.