If you post the small .ERR log, without giving any further details about your system ( for example the simulator used ), it's not very easy to help you so, at least, provide the detailed LOG, the one that can be obtained by enabling LOGGING in the Troubleshooting page of the GSX Settings.
That log is far more detailed, and might help us knowing more about the problem, lacking a better report.
IF you have logging enabled, if the error happen again, don't restart Couatl when asked ( otherwise you'll lose the log with the error ) but, instead, exit the sim and get the %APPDATA%\Virtuali\Couatl.LOG ( not .ERR ) file, and Attach it to a post, zipped.