Umberto,
it *was* in fact the same problem... It might not be clear from my first post, but I have had these sceneries registered for years, and they have worked just fine. But then, one day last week, when I booted up my FSX, they had all stopped working and were listed as Trial in the Add-On Managar.
Initially I thought that it was just a glitch, so I flew to some other airports that day.
The following day, I had the same problem. And the following day again.
I then tried to re-activate a couple of the sceneries by using the keys that I had received back when I bought the sceneries. But they did not work, as in: Nothing happened, when I inserted the keys and pressed the button.
That's when I gave up and wrote the original post on the forum.
The problem persisted yesterday.
But this morning, when I booted up my PC and FSX, everything was suddenly back to normal, and GSX compiled a new airport list when a flight was loaded.
As I had done nothing other than what I have mentioned here to correct the problem, I have no idea why the problem started, and why it suddenly ended again.
But the status right now: I'm back up and running. (I just wish I knew what happened...)
Best
/JB