When I click on a shopkeeper the game kicks me out of the server with the message:

internal exception io.netty.handler.codec.decoderexception java.lang.illegalargumentexception: attempt to get net.minecraft.network.codec.Stream.Codec field "net.minecraft.network.protocol.game.PacketPlayInUseEntity.a" with illegal data type conversion to int

The shopkeepers plugin does not modify how the server handles packets, so may guess is that this issue is caused by some other component.

Try to reproduce this on a up-to-date Spigot server, without other plugins, and without any client-side mods.

Also, check if you find a complete error stack trace in your server log that points to some other plugin on your server.