Server sometimes crashes when loading a chunk enderworld.

http://pastebin.com/7NUx0LsB

It doesn't seem to be caused by the mod, does it still happen when you backup the world and try it without the mod? If it doesn't, add a comment here with list of mods that you're running, details about version of minecraft etc. - that should be at the end of crash report (the one you put on pastebin was probably too long, that info is not there).

Possible conflict with the GregTech. But other mods influencing generation of the world is not installed.

Well, send me the part of the crash log that has all the version data and other things for MC and installed mods.