Glowstone server is generating only one strange biome!

Bukkit is not available at the moment and I searched for other server software that supports plugins. Glowstone supports the Bukkit API, so I am able to use Bukkit plugins.

But I have a problem with the world generator. I do not even think such a biome exists... As you can see the sand is fading to the grass.

Screenshot

The new generated maps always starts with rain and thunder. What do I have to do to get this working?

System Information

Glowstone: 1.8-84-gd5a1f56-b86

Java: 1.7.0_67

Client: 1.8

OS: Windows 8.1

Arch: amd64

Plugins

WorldEdit: 6.0.0

VoxelSniper: 5.170.0

I also tried it without plugins!

Configuration File

# glowstone.yml is the main configuration file for a Glowstone server
# It contains everything from server.properties and bukkit.yml in a
# normal CraftBukkit installation.
#
# For help, join us on IRC: #glowstone @ esper.net

server:
    port: 25565
    name: Glowstone Server
    online-mode: true
    max-players: 20
    whitelisted: false
    motd: Glowstone Server
    shutdown-message: Server shutting down

folders:
    plugins: plugins
    update: update
    worlds: worlds

world:
    name: world
    spawn-radius: 16
    view-distance: 8
    gen-structures: false
    allow-nether: false
    allow-end: false