Have very weird bug. Friend can't log in my LAN server ("Failed to synchronize registry data from server") if we remove or update some random mod. If we remove Quark - problem gone. Found this issue with mods like BYG, Mahou Tsukai and OptiFine (Maybe more).
World using Quark realistic generation.
Here latest log (But i think there nothing interesting): https://gist.github.com/Ibirtem/28ce2b7974cadad72e0d76a200e4c3f7
Quark and Forge latest

Make sure you and your friend have the exact same mod list with the exact same versions for each; if you do, please attach the log for your friend's mod as well.