In my experience it was due to a customisation file being bad after possibly an update. For now rename the original file so that it doesn't get picked up my GSX (add any number of characters before the airport code), then restart Couatl at the apirport and go through the gates copying the data for one gate into the new file, restarting Couatl each time to reload the new file and check its valid. When you hit the error again it means that gate data might be corrupt.
