[flying-pig]

UP can be a bit weird, first try running the auto updater, see if u can join. ( you do not need to unload the mods, but do it anyway ) ie no mods in active list.

If that fails, try a ittle trick that sometimes help, turn off all mods, turn on the stock 4.09 mod, then turn it off again, and turn on the mods u need.

It may help, best of luck!