Bungee/Waterfall To a Modded Server

[dualspiral]

It might be this if you tried using Bungee first:

Spongeforge + Bungeecord + SpongePls SOLVED

Usually happens when forge support is disabled in Bungee. Check the forge config option is set to true. I recommend using Waterfall as a drop in replacement for Bungee. You should also remove SpongePls if you switch, my patch that enables it is baked into Waterfall.