Your friend has the same issue as me and I have gone through the same steps and nothing is working. Reason I asked about that settings cuz sometimes I can’t uncheck it. Just now had to restart the game twice before it let me uncheck it.
This issue is so frustrating cuz I do group flights with friends and I can’t, very disappointing. Hopefully they can resolve this come next patch
Since day one I’ve had the same problem, only one or two planes in the air, even over nyc and lax. I contacted support and they had me try numerous settings. Nothing we tried worked. They gave up.
I see many boat labels and cars but almost no air traffic anywhere. MSFS2020 always had many players in the air and on the ground. My friends can’t see me, and I can’t see them.
What has changed, I tried loading MSFS2020 on my old and new pc again, and now it has the same empty sky problem too.
Could something like my Microsoft Store account corrupted both of these programs?
I keep seeing Error accessing live traffic data, retrying
I hope someone can figure out what’s going on.
Still having issues after today’s update
I had similar impressions at first, but I think there is a visual disappointment here that is due to the fact that the huge signs from 2020 have been removed. Back then, multiplayer traffic was shown hundreds of kilometers away and that of course creates a completely different picture than if you drastically reduce the display radius. 
Added to this is the confusing assignment of the respective online options of the UI … in many places illogical and confusing
Under “Weather and time” I have it set to present. Under “air traffic” I have it set to “live” and under “multiplayer,” I don’t have the option to click “live” or “all players.” I only have the “off” option. However, I see other aircraft on the game.
I configured everything in my settings, but I still don’t have the option for “all players” under multiplayer. Why is that?
you need to use the preset for weather
Deleted the UserCfg.opt file and let the sim regenerate a new one
Changed multiplayer servers
Delete and re-create a new rolling cache
Tried both Live Players and All Players in the flight conditions menu
Created a group flight via friends list
Completely re-install the sim (we’ve done so 3 times now)
Ran the sim with no addons
Installed the sim on a different drive
Tried using a completely different internet connection
Installed the sim on a PC that has never had the sim on it before
+1 also tried all this…
people need to stops replying to this forum with ’ make sure all players is set… use preset weather…’ doesnt work and needs to stop being repeated… this is a much bigger issue than our settings
I fully agree with you that even the “preset weather” option is not stable, it changes without our consent and in a way that seems random.