So news on this one:
Everytime restart the game (i think) it creates a new directory for the server: http://puu.sh/g3N4z/6470762a98.png
Don't now why, but happend since update, i deleted a bunch of those for some time therefore there are so 'few'.
The world-ID is normaly generated once and then stored in the level-file. It look like your "server" always generate a new world-ID when restartet. There are three reasons :
1) You added another mod to the pack by yourself wich causing it.
2) Your world is corrupted. Try with a new world by re-naming the "world"-folder to something else so it gets re-created (if 1 is true, then delete the whole mess and re-download server and client before)
3) You're using a cracked client and set server to offline-mode wich may have some unknown side-effects wich causing this bug. Solution > BUY THE GAME !