Same issue for me in EGPE and LOWI, some airports work fine, some not. Only with PMDG.
The rear stair gets stuck and then disappear after a while, and GSX crash (it says the engine hasn't started). Tried removing the door check but got the error anyway.
UPDATE: Ok found something. In EGPE and LOWI I have old profiles downloaded (for walk-in gates). If I remove the profile the stairs dock perfectly (of course I have a passenger bus now).
UPDATE 2: Only seems to work if I don't create a profile. As soon as I modify anything, the stair doesn't connect and GSX crashes.