I drop that here, just in case you face the same trouble....
It's been weeks (months?) that I couldn't access the Visual Studio Marketplace with VS2019 through the Extensions/Manage extensions menu (and I also had troubles with VSCode)
After two VS Reset and ReInstall... No more luck
Today, after checking the Firewall, changing DNS, looking at the network traffic with Fiddler... I found no issue with my laptop.
Eventually, I decided to uncheck IPv6 of my Wifi card... And Bingo!!!
Now, everything works like a charm
Cheers,
Fab.