One of club youngbloods got a fenix 7.
He's always happy with numbers he scores but (till now) he always failed to upload tracks in gp3s as they are full of errors.
I tried to check the gpx files using gpsresult and and you notice peaks (highs and lows) in the space of a few seconds. Which is probably why the site does not want to load the tracks.
He tried to synchronize the fenix with the site but it does not work...
He tried using app-pro but it is the same thing...
Is there something wrong with the device settings or what?
Does he need to reset the device?
Any help will be appreciated

Did you say .GPX file?
Then that will be the issue. You are only getting the positional data which can jump all over the place. You need to download the .FIT file and run that.

You should also check the watch is not set up for "smart" logging (which is the default) and change it to every second.

This might help you with the setup - logiqx.github.io/gps-guides/guidance/garmin/

Most common ground for this is indeed SMART logging enabled instead of ONCE per SECOND , see also www.cnet.com/tech/mobile/the-one-setting-you-should-change-on-your-garmin-watch/