Minecraft was auto-updated to version 1.12.2 yesterday and it was a great tragedy for the kids who were at home on summer vacations, while I was still at work, since after upgrade local LAN multiplayer did not work, and neither could they start the Minecraft server.
As it turns out the error was quite simple once you found it. The installer updated the game fine, but the Windows Firewall rules were not updated properly. I’d guess it’s because the kids are not using admin accounts on their computers.
On the first computer, the Windows Firewall rules were added for
but only enabled for java.exe, not for javaw.exe.
On the second computer, there was only added a rule for
…\minecraft\runtime\jre-x64\1.8.0_51\bin\javaw.exe, but it wasn’t enabled.
(I assume the difference comes from the first computer also running the Minecraft server.)
Additionally, the firewall rules for the old version,
…\minecraft\runtime\jre-x64\1.8.0_25\bin\javaw.exe, were left still enabled.
Cleaning up the rules solved the problems of peer-to-peer play. Updating the connection info to the Minecraft server to its new IP-adress, plus updating the launch path for java.exe solved the remaining glitch.
The world can start turning again for the kids.