See Screenshot for the full error message.

I dont know why this happens

Restarting does not work. We had this error one time before. We just had to wait a day and restart...

Server log:
2012-11-17 19:09:05 [INFO] Generating keypair
2012-11-17 19:09:14 [INFO] Starting Minecraft server on 176.65.141.231:5522
2012-11-17 19:09:16 [INFO] Starting minecraft server version 1.4.4
2012-11-17 19:09:16 [WARNING] To start the server with more ram, launch it as "java -Xmx1024M -Xms1024M -jar minecraft_server.jar"
2012-11-17 19:09:16 [INFO] Loading properties
2012-11-17 19:09:16 [INFO] Default game type: SURVIVAL
2012-11-17 19:09:16 [INFO] Generating keypair
2012-11-17 19:09:17 [INFO] Starting Minecraft server on 176.65.141.231:5522
2012-11-17 19:09:18 [INFO] Preparing level "world"
2012-11-17 19:09:19 [INFO] Preparing start region for level 0
2012-11-17 19:09:20 [INFO] Preparing spawn area: 3%
2012-11-17 19:09:21 [INFO] Preparing spawn area: 26%
2012-11-17 19:09:22 [INFO] Preparing spawn area: 34%
2012-11-17 19:09:23 [INFO] Preparing spawn area: 47%
2012-11-17 19:09:24 [INFO] Preparing spawn area: 52%
2012-11-17 19:09:25 [INFO] Preparing spawn area: 57%
2012-11-17 19:09:26 [INFO] Preparing spawn area: 61%
2012-11-17 19:09:27 [INFO] Preparing spawn area: 65%
2012-11-17 19:09:28 [INFO] Preparing spawn area: 66%
2012-11-17 19:09:30 [INFO] Preparing spawn area: 68%
2012-11-17 19:09:31 [INFO] Preparing spawn area: 70%
2012-11-17 19:09:32 [INFO] Preparing spawn area: 73%
2012-11-17 19:09:33 [INFO] Preparing spawn area: 77%
2012-11-17 19:09:33 [INFO] Done (15,187s)! For help, type "help" or "?"
2012-11-17 19:09:33 [INFO] BangerLP [/88.72.161.206:52593] lost connection
2012-11-17 19:09:33 [INFO] /88.72.161.206:52599 lost connection
2012-11-17 19:57:30 [INFO] MrTobi888 [/178.25.84.67:52071] lost connection
2012-11-17 19:57:39 [INFO] MrTobi888 [/178.25.84.67:52078] lost connection
2012-11-17 19:57:56 [INFO] NicoHood [/91.53.136.194:60268] lost connection
2012-11-17 19:58:44 [INFO] /178.25.84.67:52129 lost connection