What version of ViaVersion are you using? Type /ver viaversion
What version of Spigot are you using? Type /ver
What plugins are you using? Type /plugins
Are you using any additional software like BungeeCord? If so, what software and version? (Please list your plugins as well)
How does this error happen? login? Using an item?
Is there an error in the console? Use pastebin.com. Is there a kick message?