Updated to most recent pack version, S.O. was trying to join [ We've been playing fine for a while. ] And they get this error.

We've tried another world, changing versions of Neo Forge as well as using same config files.

As a note, I'm hosting and am able to play normally

same issue for me and my son. unable to disable supplimentaries on my sons pc as FTB wont load mod list. disabled on mine and will test.

Adding to this, I was having the same issue; removing Supplementaries fixed it.

Adding to this, I was having the same issue; removing Supplementaries fixed it.

That works!