If it happens only with the PMDG MD11 Freighter, check all the following:
- You use the most current version of GSX, which means you have either installed all the Live Updates OR you installed GSX with the Full 1.8 installer
- You use the configuration that comes with GSX, not an older configuration you might have created for the previous version. You can check this using the "Reset" option in the GSX Airplane configuration editor. And, check you also don't have any GSX.CFG files in the FSX\Simobjects\Airplanes\PMDG MD11F folder
If the error still happen, try to enable logging in the "Troubleshooting" section of the "GSX Settings" menu, and check if a Couatl.ERR file is generated in your %APPDATA%\Virtuali folder. If yes, post its content here.