[17:26:18 INFO]: Keendy issued server command: /nick &dsa
[17:26:18 WARN]: Missing translation key "nickNamesOnlyColorChanges" in translation file ru
java.util.MissingResourceException: Can't find resource for bundle java.util.PropertyResourceBundle, key nickNamesOnlyColorChanges
at java.util.ResourceBundle.getObject(ResourceBundle.java:450) ~[?:1.8.0_101]
at java.util.ResourceBundle.getObject(ResourceBundle.java:444) ~[?:1.8.0_101]
at java.util.ResourceBundle.getString(ResourceBundle.java:407) ~[?:1.8.0_101]
at com.earth2me.essentials.I18n.translate(I18n.java:64) [EssentialsX-2.0.1.jar:?]
at com.earth2me.essentials.I18n.tl(I18n.java:77) [EssentialsX-2.0.1.jar:?]
at com.earth2me.essentials.commands.Commandnick.formatNickname(Commandnick.java:83) [EssentialsX-2.0.1.jar:?]
at com.earth2me.essentials.commands.Commandnick.run(Commandnick.java:35) [EssentialsX-2.0.1.jar:?]
at com.earth2me.essentials.commands.EssentialsCommand.run(EssentialsCommand.java:146) [EssentialsX-2.0.1.jar:?]
at com.earth2me.essentials.Essentials.onCommandEssentials(Essentials.java:466) [EssentialsX-2.0.1.jar:?]
at com.earth2me.essentials.Essentials.onCommand(Essentials.java:376) [EssentialsX-2.0.1.jar:?]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at org.bukkit.craftbukkit.v1_11_R1.CraftServer.dispatchCommand(CraftServer.java:650) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.PlayerConnection.handleCommand(PlayerConnection.java:1354) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.PlayerConnection.a(PlayerConnection.java:1189) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.PacketPlayInChat.a(PacketPlayInChat.java:45) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.PacketPlayInChat.a(PacketPlayInChat.java:1) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_101]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_101]
at net.minecraft.server.v1_11_R1.SystemUtils.a(SourceFile:46) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:747) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:399) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:678) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:576) [spigot.jar:git-Spigot-3fb9445-fecf4b4]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_101]

there must be an error in messages_ru.properties
[17:26:18 WARN]: Missing translation key "nickNamesOnlyColorChanges" in translation file ru
java.util.MissingResourceException: Can't find resource for bundle java.util.PropertyResourceBundle, key nickNamesOnlyColorChanges
Make sure you changed the lang file in config from en to ru. Check the file for any errors.

The error doesn't help us. Please tell us exactly how to reproduce the problem, so we can replicate it and identify it properly. Create a new issue if you are still having this in the latest version of Essentials.