Bukkit

What were you doing that caused the problem?
Logging in with Spoutcraft client

What's the error? Post using bukkitPaste.
http://dev.bukkit.org/paste/5226/

What version of SpoutEssentials are you using? (Look in the console, not the thread)
R: SpoutEssentials for CB 1.2.5-R1.0

Post your plugin list. (A screenshot of '/plugins' would do)
Uploaded screenshot

Post all of your configs in spoutessentials folder, using bukkitPaste.
http://dev.bukkit.org/paste/5227/

BukkitPaste can be found here:
http://dev.bukkit.org/paste/

You need world guard or else you'll get that error. If you don't want world guard support, go into modules and disable the world guard section.

I have WorldGuard.

I have WorldGuard support turned on in my modules.yml

Why's it doing this?

PS. I updated my plugin list.

The error is shown right there:

Caused by: java.lang.IllegalArgumentException: A Texture Pack must be in a .zip format

You get that when a Spoutcraft player joins, because it's trying to send them whatever you have set as a texture pack, but you don't have a valid texture pack zip set.