Relevant log: https://pastebin.com/X72P9k4x

https://github.com/MinerArcana/Astral/blob/develop/1.16.x/src/main/java/com/alan19/astral/blocks/IndexOfKnowledge.java#L62
This is probably the issue, if the player is on a server, it sends a blank packet which will always fail with a null pointer exception when it tries to encode the packet.
Works in single player because minecraft skips the packetbuffer encode/decode