I believe the problem has been found, thanks to the help from Joggu, which I had two separate sessions with.
After trying everything, we found that, on his system, the add-on.xml path for a scenery from FSDG resulted in a crash when being read. It was related to the fact his user name contained non-ASCII characters, but the strange thing was, ALL the other sceneries installed ( even others from FSDG ), had their add-on.xml in the same folder, since his user name it's always the same.
But for some reason, this particular add-on.xml couldn't be opened. The file itself was fine, it was just the problem of being in a path with a non-ASCII username, but the thing that puzzles me is all the other add-on.xml were in the same path.
This triggered some kind of bug in an open-source library we use to parse xml file, which made the Addon Manager crash, and the sim with it ( a .DLL that crashes will crash the sim, unfortunately ).
I still not fully understand what only *that* file couldn't be opened without crashing, but I managed to add a workaround to prevent this case. It's on the Live Update now, waiting to hear your feedback.
