Hi guys, suddenly my windows says me that coault.exe has no a valid certificate so i cannot run GSX on my FSX due to this problem.
That's because something has modified the executable on your system, corrupting the certificate. The whole point of having digitally-signed files, is that you will know if a file is not the same as it's supposed to be, and something on your system has changed it.
The most likely cause is the antivirus has corrupted the file, mistakenly detecting the executable as a thread and trying to "heal" it.
As explained in many other threads, you should install with the antivirus disabled, and configure the antivirus to exclude the following files from scanning before launching FSX:
YourFSXFolder\bglmanx.dll
YourFSXFolder\fsdreamteam\couatl\Couatl.exe