It seems like the problem doesn't happen with default aircraft. But obviously i don't just want to fly those.
No, the problem likely happens with only a *specific* 3rd party addon. Have you tried to customize its configuration for it ? Or, was an unsupported airplane that you made the configuration from scratch ?
I can't get the error file, I checked the setting but it doesn't show up in the file.
As I've said, the Couatl.LOG file is created
only if you enable Logging from the Troubleshooting section of the GSX Settings page. The Couatl.ERR file is created in any case, if there's an error, but contains only the error text.
So, in general, if the Couatl.ERR is missing, it's best to enable Logging so, at least, even if there's no logged error, we can see other information about your config, the airplane used, the airplane customization file, the scenery used, etc.