(sorry if old, I didn't find this particular topic with a quick search).
For a few weeks my Windows PC's Garmin Connect had some issues: no 'All Stats', no 'Splits', I couldn't access lots of heart rate, power or speed/pace data. I had to rely on my Garmin devices or on my phone's Connect app to get that data. This both on Firefox and Chrome.
The culprit was Avira Browser Safety, my antivirus software's browser extension. Once disabled/uninstalled, Garmin Connect fully works.