Server error

So I know what is causing this problem but someone please tell me how to resolve it! It's been working fine for a month but suddenly everytime I join this error occurs.

[16:48] [pool-6-thread-1/ERROR]: [PermissionsEx] Error occurred with PermissionsEx! Please post it to https://github.com/PEXPlugins/Permissio ... 8b19%29%0A. Full error:
java.lang.RuntimeException: Default user group is not defined. Please select one using the "default: true" property
at ru.tehkode.permissions.backends.FileBackend.getDefaultGroup(FileBackend.java:143)
at ru.tehkode.permissions.PermissionManager.getDefaultGroup(PermissionManager.java:294)
at ru.tehkode.permissions.PermissionManager.getDefaultGroup(PermissionManager.java:283)
at ru.tehkode.permissions.PermissionUser.getGroups(PermissionUser.java:219)
at ru.tehkode.permissions.PermissionUser.getOption(PermissionUser.java:130)
at ru.tehkode.permissions.PermissionEntity.getOptionBoolean(PermissionEntity.java:294)
at ru.tehkode.permissions.PermissionEntity.initialize(PermissionEntity.java:53)
at ru.tehkode.permissions.PermissionUser.initialize(PermissionUser.java:51)
at ru.tehkode.permissions.ProxyPermissionUser.initialize(ProxyPermissionUser.java:42)
at ru.tehkode.permissions.PermissionManager.getUser(PermissionManager.java:121)
at ru.tehkode.permissions.PermissionManager.getUser(PermissionManager.java:138)
at ru.tehkode.permissions.bukkit.SuperpermsListener.updateAttachment(SuperpermsListener.java:47)
at ru.tehkode.permissions.bukkit.SuperpermsListener.onPlayerLogin(SuperpermsListener.java:131)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:292)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:501)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:486)
at net.minecraft.server.v1_7_R4.PlayerList.attemptLogin(PlayerList.java:386)
at net.minecraft.server.v1_7_R4.LoginListener.c(LoginListener.java:68)
at net.minecraft.server.v1_7_R4.LoginListener.a(LoginListener.java:42)
at net.minecraft.server.v1_7_R4.NetworkManager.a(NetworkManager.java:160)
at net.minecraft.server.v1_7_R4.ServerConnection.c(SourceFile:134)
at net.minecraft.server.v1_7_R4.MinecraftServer.v(MinecraftServer.java:667)
at net.minecraft.server.v1_7_R4.DedicatedServer.v(DedicatedServer.java:258)
at net.minecraft.server.v1_7_R4.MinecraftServer.u(MinecraftServer.java:558)
at net.minecraft.server.v1_7_R4.MinecraftServer.run(MinecraftServer.java:469)
at net.minecraft.server.v1_7_R4.ThreadServerApplication.run(SourceFile:628)