My wife and I are trying to play SS2 together in co-op mode. It actually worked (without any additional software, like Hamachi) the first time we played the game, but it hasn't worked since.

We even tried making a new game, just in case something got corrupted, but it still wouldn't work. We haven't changed anything and we don't have any mods installed. It just stopped working when the exact same setup worked perfectly before. It makes no sense!

Does anyone here have any idea what we can do to fix this?

Please don't tell us to install and use something like Hamachi. We don't want to do that and shouldn't have to because the game initially worked in co-op perfectly the first time we played. So it's entirely possible for this game to work in co-op without necessitating the use of third-party software.

yep, that's the multiplayer for you.

make sure you are using the correct ip addresses (a modern pc might have several and there is no guarantee the game will see the correct one). and hope for the best.

We checked our router (for our IP addresses), so we know those are correct. We initially hosted the game on my wife's machine, but just to see if it'd work, we tried hosting it on mine this time, but no luck.

We also made an exclusion in our virus scanner for SS2, but that didn't help either. Turning off the virus scanner didn't make a difference.

This is so weird. Why would it work the first time, but not after that? Nothing's changed (and yes, we checked to see if our IP addresses changed, but they didn't) during the time between our first successful attempt and all following failed attempts.

that's how things are - I'm also unable to run it properly if that's of any consolation, while I usually am able to connect to the host machine, synchronization issues will break things upon the next level load with 99% chance.

if not being able to see the other computer is the problem, I would suggest using a crosswired lan cable and setting a fixed ip address.

Maybe its stucked at firewall, is it closed ? if it's not, did you try add extra rules ?

Also are these computers connected network with cable or wireless ? Sometimes wireless protocols are gettin crazy and they lost local network definitions. I mean sometimes my devices that uses wireless connection can't see my shared folders and at these times i have to reset my wps on my modem.