2nd use of new device out of the box - fully updated.
Took a 40 mile cycle route (generated by me in Garmin Connect). At the 25 mile stage, the route looped back on itself for about a mile.
When I turned back, the device kept saying "take a U-turn". Then went through a few messages saying "course lost". Then totally reset.
When it came back alive, the route was still active but surrounded by a red box (which I now think I know was telling me recording was paused?)
As I was descending at 40mph+ I didn't want to mess!
Eventually I managed to get recording restarted without losing the previous 25 miles of data.
My recorded data ended up losing 2 miles of the trip (during the descent).
I include the error log, the course and the log data (.fit files) if it's of any help...