A possible reason might be your EXE.XML being reverted to a previous version (one before you installed GSX), usually caused by another add-on that handles it and might have restored it to a previous version, or you might have just lost the file, or it might gone corrupted.
A first try would be Unlinking GSX Pro, meaning you select Uninstall, and reply YES when it asks to Unlink it, but NO when it ask to Uninstall it. Then, after the red "Relink" button shows up, click it again, to Relink it, so it goes back to "Installed" in green.
This will try to recreate the Couatl engine lines that might be missing in your EXE.XML for any reason.
