Internal Server Error on login

Occasionally whenever someone logs in, they get an Internal Server Error:

Code:

10:03:18 [WARNING] Failed to handle packet: java.lang.NullPointerException
java.lang.NullPointerException
        at net.minecraft.server.ServerConfigurationManagerAbstract.attemptLogin(ServerConfigurationManagerAbstract.java:238)
        at net.minecraft.server.NetLoginHandler.d(NetLoginHandler.java:123)
        at net.minecraft.server.NetLoginHandler.c(NetLoginHandler.java:45)
        at net.minecraft.server.DedicatedServerConnectionThread.a(DedicatedServerConnectionThread.java:44)
        at net.minecraft.server.DedicatedServerConnection.b(SourceFile:29)
        at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:579)
        at net.minecraft.server.DedicatedServer.r(DedicatedServer.java:215)
        at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:496)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:428)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:818)

Plugins:

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