"Could not find the main class" error after stopping server

Hi!
i have a strange problem with my crafbukkit. The server was running but i had to restart it (some lags etc). It always worked, but now, i cant restart it. It just says "Could not find the main class: org.bukkit.craftbukkit.Main." Its the same file that was running a few seconds ago. I didnt change anything, i just stopped the server with the stop command.

I also tried to reinstall java, but it didnt help.

My server is running on Ubuntu server 11.04 x64 (2.6.38-10-server) with java version "1.6.0_26"

greets

 

After a Server restart I was able to start craftbukkit again, but always when I stop craftbukkit i have to restart the server, otherwise I cant restart it again. Does someone know what this causes?​

I cant always restart the server.​