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I was trying to start my server with Pixelmon and Lucky Blocks installed, but it didn't work. When I moved Pixelmon out of the mods folder it worked.
Crash Report:

---- Minecraft Crash Report ----
// I'm sorry, Dave.

Time: 4/8/15 7:22 PM
Description: Exception in server tick loop

cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraftforge/event/world/BlockEvent$PlaceEvent
at cpw.mods.fml.common.LoadController.transition(LoadController.java:162)
at cpw.mods.fml.common.Loader.preinitializeMods(Loader.java:515)
at cpw.mods.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:88)
at cpw.mods.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:317)
at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:170)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:620)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NoClassDefFoundError: net/minecraftforge/event/world/BlockEvent$PlaceEvent
at com.pixelmonmod.pixelmon.config.PixelmonItemsBlocks.load(PixelmonItemsBlocks.java:28)
at com.pixelmonmod.pixelmon.config.PixelmonItems.load(PixelmonItems.java:221)
at com.pixelmonmod.pixelmon.config.PixelmonConfig.loadConfig(PixelmonConfig.java:160)
at com.pixelmonmod.pixelmon.Pixelmon.preInit(Pixelmon.java:105)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:513)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at cpw.mods.fml.common.LoadController.sendEventToModContainer(LoadController.java:208)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:187)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:118)
at cpw.mods.fml.common.Loader.preinitializeMods(Loader.java:513)
... 5 more
Caused by: java.lang.ClassNotFoundException: net.minecraftforge.event.world.BlockEvent$PlaceEvent
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
... 36 more
Caused by: java.lang.NullPointerException

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
Minecraft Version: 1.7.10
Operating System: Linux (amd64) version 3.13.0-24-generic
Java Version: 1.7.0_67, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 110225504 bytes (105 MB) / 257425408 bytes (245 MB) up to 477102080 bytes (455 MB)
JVM Flags: 1 total; -Xmx512M
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v9.05 FML v7.10.25.1207 Minecraft Forge 10.13.0.1207 4 mods loaded, 4 mods active
mcp{9.05} [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed->Pre-initialized
FML{7.10.25.1207} [Forge Mod Loader] (cauldron-1.7.10-1.1207.01.198-server.jar) Unloaded->Constructed->Pre-initialized
Forge{10.13.0.1207} [Minecraft Forge] (cauldron-1.7.10-1.1207.01.198-server.jar) Unloaded->Constructed->Pre-initialized
pixelmon{3.4.0} [Pixelmon] (Pixelmon-1.7.10-3.4.0-universal.jar) Unloaded->Constructed->Errored
Profiler Position: N/A (disabled)
Is Modded: Definitely; Server brand changed to 'cauldron,craftbukkit,mcpc,fml,forge'
Type: Dedicated Server (map_server.txt)

Why does it work with Pixelmon 3.3.8 and not 3.4.0?

Thank you