I plugged out my Lan cable, startet the game. Nothing happened. Why?

Then i plugged it back in and the game starts. Why?

Alright, I just tested and could reproduce the issue, but noticed that it only happens with the Classic version, so you should at least be able to play the other one (which isn't able to connect to Battle.net).

Still, it's a bit weird, since Diablo "Classic" also has offline mode, it should be able to start without network connectivity.

Thank you for testing and reproducing the issue.

I had a similar issue with Diablo 2. Basicaly the Windows Game Explorer checks for updates of old games. Without an internet connection nothing happens and the game won't launch.

This fixed my problem and might also work with Diablo 1:
run regedit.exe
change the key HKEY_CLASSES_ROOT\LocalSettings\Software\Microsoft\Windows\GameUX\ServiceLocation\Games to any local folder or location.

I got the solution from here:
superuser.com/questions/805170/windows-7-when-running-old-games-rundll32-is-started-for-the-first-time/1010766 #1010766