After HOURS of fiddling around with the game (including erasing my SIX YEAR OLD SAVE FILE (INCLUDING THE CLOUD SAVE)) I have found a POTENTIAL CAUSE of this bug, and a TEMPORARY SOLUTION that should fix the login issues until Mojang does something about this.
Basically, after playing around with the game and narrowing it down, this bug has something to do with the "Max Framerate (Experimental)" option in the video tab of the settings menu.
HOW TO FIX YOUR GAME IF YOU CAN NOT LOG IN TO NINTENDO'S SERVERS:
Start Minecraft. Wait fifteen seconds for the game to attempt to log into Nintendo Network.
Put your Nintendo Switch console into airplane mode by holding down the home button and toggling airplane mode from the quick menu. Close the quick menu.
Back in Minecraft's title screen, open the settings menu. Navigate to the Video tab.
Switch "Max Framerate (Experimental)" to the UNLIMITED option (all the way to the right)
Back out to the main menu.
Close the game. Turn airplane mode off.
Restart the game. You may have to re-log into your Microsoft Account by going to aka.ms/remoteconnect.
This SHOULD fix the problem. It did for me.
After HOURS of fiddling around with the game (including erasing my SIX YEAR OLD SAVE FILE (INCLUDING THE CLOUD SAVE)) I have found a POTENTIAL CAUSE of this bug, and a TEMPORARY SOLUTION that should fix the login issues until Mojang does something about this.
Basically, after playing around with the game and narrowing it down, this bug has something to do with the "Max Framerate (Experimental)" option in the video tab of the settings menu.
HOW TO FIX YOUR GAME IF YOU CAN NOT LOG IN TO NINTENDO'S SERVERS:
Start Minecraft. Wait fifteen seconds for the game to attempt to log into Nintendo Network.
Put your Nintendo Switch console into airplane mode by holding down the home button and toggling airplane mode.
Open the settings menu. Navigate to the Video tab.
Switch "Max Framerate (Experimental)" to the UNLIMITED option (all the way to the right)
Back out to the main menu.
Close the game.
Restart the game. You may have to re-log into your Microsoft Account by going to aka.ms/remoteconnect.
This SHOULD fix the problem. It did for me.