Updated GSX and... PROBLEM SOLVED!!!
Yes, I was quite sure of it.
It's still not a solution I'm 100% happy with, because it's possible that, on some unusual configurations or unusual usage habits that for home cockpit builders or anyone with more than one video card, might still result in Render-To-Texture stuff not being rendered (you can't see any of them right now, because they'll appear with KORD 2 and GSX Level 2), for example if you start the sim with the scenery displayed on the main video card, then for some reason you drag it to another card but, that's maybe some kind of extreme example so, before trying to support something that maybe nobody use, I'll wait for some feedback first.