Can't update, console errors on server start

[VideoBoy]

I get this when I start a server with 3.1.3. My client gets stuck at 'Logging in' and nothing about the login attempt appears on the console. I don't have to reset the world for this small update, do I? I cleared the plugins folder for testing but I got the same error.

03:42:04 [iNFO] Done (1.956s)! For help, type "help" or "?"

03:42:05 [sEVERE] java.lang.NoSuchMethodError: buildcraft.api.PowerProvider.receiveEnergy(I)V

03:42:05 [sEVERE] at buildcraft.energy.TileEngine.q_(TileEngine.java:126)

03:42:05 [sEVERE] at net.minecraft.server.World.tickEntities(World.java:1189)

03:42:05 [sEVERE] at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:559)

03:42:05 [sEVERE] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:457)

03:42:05 [sEVERE] at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)

03:42:05 [sEVERE] Unexpected exception

java.lang.NoSuchMethodError: buildcraft.api.PowerProvider.receiveEnergy(I)V

at buildcraft.energy.TileEngine.q_(TileEngine.java:126)

at net.minecraft.server.World.tickEntities(World.java:1189)

at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:559)

at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:457)

at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)

BONUS: What the hell is this CCPlugin.jar thingie?