Friend can't join modded server?

Okay so, recently, my friend and I made a modded server together and we both got all of the same mods and have the same forge and everything. But, when he tried to join my server, it wouldn't let him in, so we tried again but on his server but it also didn't let him in but on both times, I was able to join. Direct connect doesn't work either. Can anyone help? :p

Failed to synchronize registry data from server, closing connection

Yeah we do.