Here's the error message in the console:
[20:37:28] [Server thread/INFO]: SimpleClans command failure: null
[20:37:28] [Server thread/INFO]: java.text.MessageFormat.applyPattern(Unknown Source)
[20:37:28] [Server thread/INFO]: java.text.MessageFormat.(Unknown Source)
[20:37:28] [Server thread/INFO]: java.text.MessageFormat.format(Unknown Source)
[20:37:28] [Server thread/INFO]: net.sacredlabyrinth.phaed.simpleclans.commands.MenuCommand.execute(MenuCommand.java:241)
[20:37:28] [Server thread/INFO]: net.sacredlabyrinth.phaed.simpleclans.executors.ClanCommandExecutor.onCommand(ClanCommandExecutor.java:130)
[20:37:28] [Server thread/INFO]: org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
[20:37:28] [Server thread/INFO]: org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141)
[20:37:28] [Server thread/INFO]: org.bukkit.craftbukkit.v1_8_R1.CraftServer.dispatchCommand(CraftServer.java:646)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.PlayerConnection.handleCommand(PlayerConnection.java:1115)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.PlayerConnection.a(PlayerConnection.java:950)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.PacketPlayInChat.a(PacketPlayInChat.java:26)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.PacketPlayInChat.a(PacketPlayInChat.java:53)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.PacketHandleTask.run(SourceFile:13)
[20:37:28] [Server thread/INFO]: java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
[20:37:28] [Server thread/INFO]: java.util.concurrent.FutureTask.run(Unknown Source)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:696)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:634)
[20:37:28] [Server thread/INFO]: net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:537)
[20:37:28] [Server thread/INFO]: java.lang.Thread.run(Unknown Source)

For me yes, though I get no stacktrace even with debug mode enabled, opped or non-opped.

@RoboMWM latest version?

Yes, but after restarting the server it... disappeared... so I'm not sure what causes it. It eventually starts happening though. Would be nice if it didn't swallow the errors though, would like it to print a full stacktrace (and I did enable debug mode, to no effect).

Is this still an issue?

@RoboMWM Do you use 1.8 as well?

No

I enabled stacktrace printing on my fork somewhere, but I haven't been able to investigate iirc... unless I'm wrong and I did put a fix in my fork. Unsure.