I didn't make it!
Whenever I attempt to start a multiplayer game, own server, solo play or otherwise, it begins to load then tells me the '.npc extensions are too long' and returns me to the main screen. Anyone know how to solve this?
Try a forum search.
You've got too many custom NPCs installed.
The only way you're going to fix it is to remove some .npc files.
I don't know how, and I don't know why, but this is totally Sheep's fault.