Put the summary of the bug you're having here

What I expected to happen was...:
Others connect to Minecraft through the LAN after opening the LAN setting 

What actually happened was...:
Everyone timed out

Steps to Reproduce:
Created a world in 1.16. It was updated to 1.17 when 1.17 was available. LAN ran fine until 29 June. Everyone could not connect through the LAN. Things tried to get the LAN to work. None of them fixed the issue.

1. Restarted Minecraft, all computers. 
2. Created new world in 1.17
3. Restarted computers

Created a 1.16.5 version option on launcher. Started up a 1.16.5 game and others could connect through LAN. 

Not sure if it is related, but Minecraft did download a 60 mb item before the issue started.