When I try to play this game multiplayer over a LAN, an error pops up during gameplay that a network error has occured, then the game goes to the main menu. I remember this being a big issue with the game back in the day, but I haven't seen any posts about this here...

Unfortunatly GOG can only get it to run on modern systems if the mulitplayer was screwed from the start in the original game thats going to be the case here. I did come across someone that claims to have a fix heres the link
[url=]http://forum.jowood.com/archive/index.php/t-152039.html[/url]
Should only need to use the intructions under the expansions gold bit.

Jowood is a terrible company. It's like a seal of fail if you see their name anywhere on a game. Multiplayer in second Guild is broken and it wouldn't surprise me if it didn't work in the first game either.