I lost quite a few hours trying to figure out why I could not host nor join a multiplayer game on EU2 to finally find out the problem was that I did not have DirectPlay installed.

DirectPlay is an old API from DirectX and on newer Windows versions you need to install/enable it from the Windows Features settings. In the Windows Features dialog look for "Legacy components" and inside you'll find DirectPlay.

Generally it enables automatically when you install some old games, but in my case it did not so I had to do it manually. If anyone else has the same problem where clicking "Host" does nothing that's why.