Started by happening every 15 minutes or so, but now the server is only up for about a minute before it happens again. (it meaning the server crashing)

Here's the console:
30.07 16:56:54 [Server] INFO at pa.run(SourceFile:714)
30.07 16:56:54 [Server] INFO at net.minecraft.server.MinecraftServer.q(SourceFile:376)
30.07 16:56:54 [Server] INFO at org.apache.logging.log4j.spi.AbstractLogger.info(AbstractLogger.java:1011)
30.07 16:56:54 [Server] INFO at org.apache.logging.log4j.core.Logger.log(Logger.java:110)
30.07 16:56:54 [Server] INFO at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:367)
30.07 16:56:54 [Server] INFO at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:406)
30.07 16:56:54 [Server] INFO at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:425)
30.07 16:56:54 [Server] INFO at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:89)
30.07 16:56:54 [Server] INFO Exception in thread "Server Shutdown Thread" org.apache.logging.log4j.core.appender.AppenderLoggingException: Attempted to append to non-started appender ServerGuiConsole
30.07 16:56:54 [Server] INFO 2014-07-30 16:56:54,995 ERROR Attempted to append to non-started appender ServerGuiConsole
30.07 16:56:54 [Server] Server Shutdown Thread/INFO Saving players
30.07 16:56:54 [Server] INFO 2014-07-30 16:56:54,994 ERROR Attempted to append to non-started appender SysOut
30.07 16:56:54 [Server] Server Shutdown Thread/INFO Stopping server
30.07 16:56:54 [Server] Server Watchdog/ERROR This crash report has been saved to: /./crash-reports/crash-2014-07-30_16.56.54-server.txt
30.07 16:56:54 [Server] Server Watchdog/FATAL Considering it to be crashed, server will forcibly shutdown.
30.07 16:56:54 [Server] Server Watchdog/FATAL A single server tick took 60.00 seconds (should be max 0.05)
30.07 16:56:53 [Server] User Authenticator #5/INFO UUID of player SpartanChez is 7c42670e-0280-48f0-9f80-14d724975def
30.07 16:56:29 [Server] User Authenticator #4/INFO UUID of player SpartanChez is 7c42670e-0280-48f0-9f80-14d724975def
30.07 16:55:54 [Server] Server thread/ERROR This crash report has been saved to: /./crash-reports/crash-2014-07-30_16.55.54-server.txt
30.07 16:55:54 [Server] INFO at java.lang.Thread.run(Unknown Source) [?:1.7.0_55]
30.07 16:55:54 [Server] INFO at net.minecraft.server.MinecraftServer.run(SourceFile:446) [minecraft_server.14w31aBuild1406734685.jar:?]
30.07 16:55:54 [Server] INFO at net.minecraft.server.MinecraftServer.x(SourceFile:530) ~[minecraft_server.14w31aBuild1406734685.jar:?]
30.07 16:55:54 [Server] INFO at pk.y(SourceFile:303) ~[minecraft_server.14w31aBuild1406734685.jar:?]
30.07 16:55:54 [Server] INFO at net.minecraft.server.MinecraftServer.y(SourceFile:614) ~[minecraft_server.14w31aBuild1406734685.jar:?]
30.07 16:55:54 [Server] INFO at qi.a(SourceFile:163) ~[minecraft_server.14w31aBuild1406734685.jar:?]
30.07 16:55:54 [Server] INFO at java.util.HashMap$KeyIterator.next(Unknown Source) ~[?:1.7.0_55]
30.07 16:55:54 [Server] INFO at java.util.HashMap$HashIterator.nextEntry(Unknown Source) ~[?:1.7.0_55]
30.07 16:55:54 [Server] INFO java.util.ConcurrentModificationException
30.07 16:55:54 [Server] Server thread/ERROR Encountered an unexpected exception