+xThis message occasionally appears if there is an invalid character in your flightplan or it is otherwise syntactically wrong. This is quite difficult to do in SkyDemon; you'd need to show us a screenshot of the flightplan which won't file and we might be able to spot what's wrong.
Thanks for all your help and the email sent - really appreciate it. I've sent some pics of the plan I was trying to file, hopefully, this might help (im going to guess a user error?)