I get this error log when trying to command mobs.
I hope you can fix this
here it is:

2012-02-05 11:37:47 [SEVERE] Could not pass event org.bukkit.event.entity.EntityDamageByEntityEvent to MobRider
org.bukkit.event.EventException
    at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1026)
    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:57)
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:453)
    at net.minecraft.server.EntityHuman.attack(EntityHuman.java:786)
    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:942)
    at net.minecraft.server.Packet7UseEntity.handle(SourceFile:33)
    at net.minecraft.server.NetworkManager.b(NetworkManager.java:226)
    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:100)
    at org.getspout.spout.SpoutNetServerHandler.a(SpoutNetServerHandler.java:587)
    at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
    at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:537)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:435)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.GeneratedMethodAccessor81.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1024)
    ... 12 more
Caused by: java.lang.NullPointerException
    at com.edwardhand.mobrider.models.Ride.<init>(Ride.java:45)
    at com.edwardhand.mobrider.utils.MRHandler.getRide(MRHandler.java:39)
    at com.edwardhand.mobrider.listeners.RiderDamageListener.onEntityDamage(RiderDamageListener.java:60)
    ... 16 more

It's only huge since the error is repeated over and over. I edited your report down to just one.

What version of bukkit are you using? You should be able to see this in the server.log.

I think I'm getting the same error: Here is the log; http://pastie.org/3326545

Bukkit ver: 1846 1.1-R3
Vault: 1.2.8
Spout: 721

Software versions:

Bukkit: 1.1-R4
Vault: 1.2.8
Spout:737

Make sure you're using a recommended build: http://ci.bukkit.org/job/dev-CraftBukkit/1846/

I'll try that

Nope not working

Pluginlist:
AnCasino
Attk
BookWorm
BouncyBeds
Buycoupon
CFBanner
Chatmanager
ChestShop
CouponCodes
Courier
Craftbookcircuits
CraftbookCommon
Craftbookechanisms
CraftBukkitUpToDate
CreativeKill
Dragonshouts
Dreamland
DrugMeUp
EggCatcher
Essentials
Fortification
HatMe
HEroicDeath
Iconomy
Jail
Launchme
Magicspells
MobArena
MobBount
MobDisguise
Mobhealth
MobRider
Modifyworld
Multivers-core -portals -Signportals
NoCheat
Orebfuscator
Permissions
PermissionsEx
pvpArena
Register
ScrollingMenuSign
Signcolours
SilkierTouch
SkylandsPlus
Snowballer
Spout
TimTheEnchanter
Turnstile
VanishNoPacket
Vault
WorldEdit
WorldGuard
Worldiventories

SSile Maybe you can post a list of plugins that you have and then remove the ones I dont have.

And I forgot to say that I have MobRider 0.45-b38

Plugins I share with you are:

BookWorm
Courier
Essentials
Iconomy
MobRider
Multiverse: COre, Portals
Nochaat
Register
ScrollingMenuSigns
Spout
Vault
WorldEdit
Worldguard.

I guess ill just test it the hard way, plugin by plugin...

I will remove everything one by one, I think it is spout

Wow I tried it on an entirely empty server... It didnt work.... Just MobRider (latest dev) and Vault (1.2.8)

I have a similar setup setup but no error.

craftbukkit 1.1-R3
Spout 721
Vault 1.2.8
BOSEconomy v0.7.2
MobRider 0.45.1-b39

might be iconomy then
EDIT: I'm gonna test right now
EDIT: Still doesnt work...
EDIT: Gonna try new dev built
EDIT: Nope

So any mob commands cause this error for you? Still not able to repro for myself.

Yeah tried latest dev version still wont work :/
I think I will retry at new RB