Internal Exception

First fix

Try to update to the latest ProtocolLib and if you use ViaVersion also update it.

Second fix (if required)

Internal Exception: io.netty.handler.codec.DecoderException: java.lang.IndexOutOfBoundsException: index: 5, length: 13632 (expected: range(0,30))

Open config.yml of ItemsAdder and change this value to false: optimized-packets: false

config.yml

  custom-entitites:
    optimized-packets: false