Happen from time to time, usually it is only to start again and it will sync on second attempt. I am on sw 5.10
I have the same problem (Code 0500) and it happened right after I synched via Garmin Express. The main firmware did not update during the update only maps and other things.
Any suggestions besides restarting the watch and synching via the phone?
Fenix 5x Plus, SW 6.00