I'v just finished a big reinstall of everything as my PC had some issues last week that caused me to need to reinstall windows.
I'v just installed the UK2000 scenery (EGLL and EGCC) that I have, and also some of FSDreamTeam's stuff in the USA that I have (KORD/KJFK).
Additionally I installed all of my Australian stuff (FTX, Orbx, OZx, Ants Aussie Airfields, couple of others).
All of the Australian stuff is working perfectly.
NL2000 (Netherlands) seems to be working fine also.
It's the UK and USA stuff that's got me scratching my head.
With EGLL I'm getting what looks like the Default scenery's positioning of aircraft. What happens now is
1: All of Terminal 5 at EGLL is devoid of any aircraft
2: All the airlines are parking in random order
3: There's stuff parking in T3,
4: There's stuff parking in the middle of buildings with their nose stuck out, wings stuck into and tails buried right in the middle of various buildings, and the odd aircraft parked at a random angle with bits overhanging taxiways.
5: the wierd one: it seems all the default scenery's Aerobridges are still there. I'v even seen a random aerobridge fouling taxiway A in some places.

EGCC has the same issues. Default Aerobridges and AI aircraft parked inside buildings or so far back from their positions they are fouling taxiways.
Used Both these airports for years and they have gone through 2 full installations before now without issue, just this time both UK and USA have shown up with these issues, and I'v reinstalled EGLL twice in the last 24 hours with no change.