Bukkit Socket Problem?

When I start my server, I am running the same plugins as yesterday, but now I get these messages:

11:19 AM [SEVERE] java.net.SocketException: Socket closed
11:19 AM [SEVERE] at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:99)
11:19 AM [SEVERE] at java.net.SocketOutputStream.write(SocketOutputStream.java:136)
11:19 AM [SEVERE] at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
11:19 AM [SEVERE] at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
11:19 AM [SEVERE] at java.io.DataOutputStream.flush(DataOutputStream.java:106)
11:19 AM [SEVERE] at net.minecraft.server.NetworkWriterThread.run(SourceFile:103)
Then it spams:

11:21 AM [WARNING] Can't keep up! Did the system time change, or is the server overloaded?

What can I do to fix this?

 

Not a bukkit problem, nothing to do with bukkit in your stack trace.

 

K, then what is causing this problem?

 

It's the vanilla minecraft server - not something we are able to help you with.