I really appreciate how frustrated you are with this error msg. First the disclaimer! I do not possess any amount of technical expertise but I did encounter the exact same message with the problem being a .dll on the FSL A320 aircraft. Here is an excerpt from Flightsim Labs FAQ that corrected the problem for me (using FSX-SE). Perhaps Umberto could comment about the possibility that a Viruali reg key has become corrupted in FSX?
"Q: I am getting the "Flight Simulator has detected a problem with a third-party software program (add-on): Name FSLSounds.dll (or FSLOptions.dll or FSLSpotlights.dll): Do you want to run this software (not recommended)? Then, even if I click Yes, Flight Simulator crashes.
A: This is because something caused an error in a previous load of Flight Simulator and Windows decided to intervene and permanently mark that DLL as unsafe. You need to run "regedit", find the registry setting for
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers
and in there, edit the entry for fsx.exe to REMOVE any mentions of
Data = $IgnoreFreeLibrary<FSLSounds.dll>
(or the other FSL dlls accordingly).
Then, restart FSX - you'll get the error message once again, but click Yes, and the application will work correctly this time.
Also, make sure your FSX is running as Administrator.
Again, I have no idea if its the same problem but it sure sounds like it.