Possible Fixes

Yes

Modpack Version

0.43

What happened?

There were version mismatch between client and server, some mods on the server side were not updated and so i've manually updated them. Here are the mods that i've updated:

Mekanism
MekanismGenerators
MekanismTools
chipped
disenchanting_table_merged
pneumaticcraft-repressurized

after updating those mods on the server side i get:
Internal Exception: io.netty.handler.codec.DecoderException: Failed to decode packet 'clientbound/minecraft:custom_payload'
just in case i tried to start a singleplayer world and i had no issue doing so.

latest.log

https://gist.github.com/Jerbear0/9c40fc815ea1b94e13b022f67b15d6a9

Developer reports

No response

turns out i was stuck between 2 updates, i had started the server just before update 0.43 and when i installed the client it had updated.