I see stuff referencing Rei's minimap still. Check the Mods folder and the Config folder for anything related to Rei's. What type of server are you running? As in, are you using MCPC plus, vanilla, ect? That error used to happen with some MCPC plus builds when using certain mods. Not sure if it still does. If you are not using MCPC plus, you might want to give it a try. It effectively integrates Forge, Bukkit, and Spigot. You can get it here: http://ci.md-5.net/job/MCPC-Plus/
If you chose to try it, build 589 is recommended. There will be three downloads, pick the top one. Put it in its own folder, and rename it to server.jar. Open notepad and type the following "java -ms512M -mx1G -jar server.jar". You may want to change the memory min (ms) and max (mx) depending on your hardware. Save it as Launcher.bat to the same folder you put the jar in. Run it and it will set everything up for you.
My only other suggestion would be to add in mods one at a time to see what is causing the error. When adding in a certain mod causes it to crash, try taking out other mods to see if its just that mod, or the mod in combination with something else. If one mod depends on another, both can be added at the same time.