[09:33:26 WARN]: Exception in thread "pool-3-thread-70"
[09:33:26 WARN]: org.apache.commons.lang.UnhandledException: Plugin ServerSigns v4.2.1-DEV generated an exception while executing task 48
        at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()Ljava/util/Collection;
        at de.czymm.serversigns.Metrics.postPlugin(Metrics.java:335)
        at de.czymm.serversigns.Metrics.access$400(Metrics.java:68)
        at de.czymm.serversigns.Metrics$1.run(Metrics.java:244)
        at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftTask.run(CraftTask.java:53)
        at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
        ... 3 more

重装服务端，或者删除serversign插件

出错的线程是插件向外发送使用报告的线程，完全可以无视它

删除 ServerSigns插件即可