Having similar issues. Sim just freezes while loading in to some airports when I try to start a flight. Around 1/2 to 2/3 in to loading.
I have London Landmarks and I can spawn at EGLL, EGKK and other non 3rd party airports but,
I also have EGTR, EGLC, EGSG from Orbx and I can't start any flight starting from those airports. I can fly in to them though if I start from another airport.
I've bought them all from Marketplace(also for UK; EGNM, EGKA from Orbx works, other 3rd party sceneries around London area also works. All bought from Marketplace).
Tried deleting the files mentioned above, closed roll cache, deleted the whole sim and components and made a clean install and nothing solved the issue.