Not to make my first post a "me too!" post, but I'm having the exact same issue as OP, also with Biomes o' Plenty. I've followed all the instructions, and done clean installs on both a PC and a Mac, but same issue in both places.
Incidentally, if I create a Biomes o' Plenty world and swap it for the "world" created by the server, the server launches, but with some "[sEVERE] no item id" warnings, and the resulting world is missing all of the unique Biomes o' Plenty blocks.
It seems like the forge server just isn't loading the mod... Any ideas why not?