I've been having the same issue (though more with TCX) intermittently over the course of the past 2 weeks or so. I never had this issue in 2 years prior, all on the same devices.
I've been getting the same for the last few days as well. Always on the GPX export and most of the time with the TCX export. I submitted a Problem Ticket on the Garmin Customer Support website. I hope they get this problem resolved soon.