i use OpGuard and this happens when i type /v:
[17:26:38 ERROR]: Unhandled exception executing command 'v' in plugin VanishNoPa
cket v3.20
org.bukkit.command.CommandException: Unhandled exception executing command 'v' i
n plugin VanishNoPacket v3.20
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spi
got.jar:git-Spigot-de459a2-51263e9]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) [spig
ot.jar:git-Spigot-de459a2-51263e9]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:14

1. [spigot.jar:git-Spigot-de459a2-51263e9]
   at org.bukkit.craftbukkit.v1_10_R1.CraftServer.dispatchCommand(CraftServ
   er.java:647) [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.PlayerConnection.handleCommand(PlayerCo
   nnection.java:1358) [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.PlayerConnection.a(PlayerConnection.jav
   a:1193) [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.PacketPlayInChat.a(PacketPlayInChat.jav
   a:45) [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.PacketPlayInChat.a(PacketPlayInChat.jav
   a:1) [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.PlayerConnectionUtils$1.run(SourceFile:
2. [spigot.jar:git-Spigot-de459a2-51263e9]
   at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [
   ?:1.8.0_73]
   at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_73]
   at net.minecraft.server.v1_10_R1.SystemUtils.a(SourceFile:45) [spigot.ja
   r:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.MinecraftServer.D(MinecraftServer.java:
3. [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.DedicatedServer.D(DedicatedServer.java:
4. [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.MinecraftServer.C(MinecraftServer.java:
5. [spigot.jar:git-Spigot-de459a2-51263e9]
   at net.minecraft.server.v1_10_R1.MinecraftServer.run(MinecraftServer.jav
   a:567) [spigot.jar:git-Spigot-de459a2-51263e9]
   at java.lang.Thread.run(Unknown Source) [?:1.8.0_73]
   Caused by: java.lang.ClassCastException: com.rezzedup.opguard.wrapper.GuardedPla
   yer cannot be cast to org.bukkit.craftbukkit.v1_10_R1.entity.CraftPlayer
   at org.bukkit.craftbukkit.v1_10_R1.entity.CraftPlayer.hidePlayer(CraftPl
   ayer.java:896) ~[spigot.jar:git-Spigot-de459a2-51263e9]
   at org.kitteh.vanish.VanishManager.toggleVanishQuiet(VanishManager.java:
6. ~[?:?]
   at org.kitteh.vanish.VanishManager.toggleVanishQuiet(VanishManager.java:
7. ~[?:?]
   at org.kitteh.vanish.VanishManager.toggleVanish(VanishManager.java:197)
   ~[?:?]
   at org.kitteh.vanish.VanishCommand.onCommand(VanishCommand.java:45) ~[?:
   ?]
   at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spi
   got.jar:git-Spigot-de459a2-51263e9]
   ... 18 more

java.lang.ClassCastException: com.rezzedup.opguard.wrapper.GuardedPlayer cannot be cast to org.bukkit.craftbukkit.v1_10_R1.entity.CraftPlayer

Here's your problem. I see you've already created a ticket here - GuardedOperators/OpGuard#11