after startup, loop with msg "Unknown command" and crash

After server startup and said: Done (secs.)
the log loop with msg: [Server thread/INFO]: Unknown command. Type "/help" for help.
and then crash

older version(>= 1.7.2) have same problem.


Server:
spigot-1.7.9-R0.2-SNAPSHOT.jar
no plugin.

server.properties:
only edit: online-mode=false

script:
java:
java version "1.7.0_55"
OpenJDK Runtime Environment...

after startup, loop with msg "Unknown command" and crash