I tried to explain that here (as far that is explicable):
That’s why this issue is such a nightmare, it points to random (or maybe even not that random) things, in some cases even things that can’t technically cause something like this like textures. Example: I have a payware airport B that loads fine. If I add a small freeware airfield 220km (!) to the south, the airport doesn’t load anymore. So it looks like the small airfield is the culprit, right?
Except it isn’t - if the airfield is not there, there’s still payware airport A that doesn’t load. I can make that load only by turning down “Off-screen terrain precaching” (how can that have something to do with it?) and that’s what users experience in all variations. Besides…I (and I assume many other affected users) don’t have anything Seafront installed and this all started acting up after installing SU6.
Edit: Just so you see how much of a mindfudge this is: Airport A doesn’t load, period, except when I turn “Of-screen yada” one notch down, even with an empty community folder. Now if I disable (by editing config.xml) payware airport C whopping 580km to the south of airport A, airport A will load without the “Offscreen…” trick. I can see why people would think that airport C must be the culprit then. Of course I can spawn at airport C just fine, it just prevents airport A almost 600km away from loading.
Speaking of which, here’s more Twilight Zone: I use VR and I change the “Offscreen precaching” thing in the VR settings of course but when I’m testing if I can load into airport A I haven’t even activated the headset yet, the sim is running in 2D! Yet changing “Offscreen…” for VR lets me still reliably load the airport in 2D (and VR of course).
Anyway, if I have disabled airport C, I can even put aforementioned little airstrip into the Community folder again and airport A will still load, but if I put another small freeware airport hundreds of km away into the folder, airport A is bricked again. That would be the third add-on that could be (mis-)identified as the culprit and that behavior supports my theory that the general amount of add-ons determines how hard users get hit by this bug.
Besides the problem apparently also coming up without any add-ons - if you consider that not only airports are involved, one could summarize the issue this way: This bug potentially renders all add-ons incompatible with each other and with the host and I think that shouldn’t be ignored in Bordeaux.