Is it private home server or a rented one? Whenever you see a "cannot resolve hostname" error, it means that it cannot find the IP address associated with that name. Did you try connecting to the straight IP address and not the name?
How are all of the devices connected to the network? I sometimes have something similar when I use my laptop wirelessly instead of a wired connection on my gaming pc. Can you just connect one person at a time to the server and see if the issue still persists?
Is the MinY value always 1? Will this allow spawning of more valuable resources at higher elevations in the world? I uninstalled my previous version of tekkit and wasn't able to compare it to an older version of the configuration file...