Every time I log on to my server it kicks about 5-10 minutes later with an error that says the packet was larger than expected. I tested some of my mods and it seems to be Flan's Mod that's causing it. I'm on Forge 14.23.5.2838 for 1.12.2 and running 5.6 Beta.5 for Flan's.

All you have to do to recreate the bug should be just play the game.
Server log: https://www.dropbox.com/s/ja1fvd7ejpfv15t/serverlatest.log?dl=0
Client log: https://www.dropbox.com/s/fqll4zj5yjrx6hf/latest.log?dl=0
They were too long to put in Pastebin

Please provide at least a copy of the client log and the server log. Also if you can not tell me what i need to do in order to get kicked from my own testing server (steps to reproduce) i am most likely not able to help you

I tested some of my mods and it seems to be Flan's Mod that's causing it.

How did you test this? You seem to have a lot of mods installed and I'd appreciate being able to see this issue with only flan's mod installed if possible.

I have a feeling this may have some type of sponge incompatibility....We also get this error every now and then and pretty much the only mods I have in common with this guy are Flans and Optifine (and sponge on server)

It's also likely that the error only happens under specific circumstances. We do a lot of entity clearing but every now and then this will happen and the server will need to be restarted

Edit: 'Every now and then' is like once a week or so. I play with friends 2-3 times a week, but nothing in particular seems to happen when we get this and usually it's only for one or two people...not everyone