I am having the same problem.
I bought a server hosting package from MCProHosting yesterday, using modified Attack of the B-Team server files (I deleted a mod, added ComputerCraft, and changed from Biomes o Plenty configs). My client is identical to the server, and I can log in just fine. My friend, however, tried to log in (using a modified client as well) and it gives him a very similar message. But his message says he is missing a ton of MineFactory mods!
But, it gets better. He is logged into Minecraft on my other computer here at home, so I was able to do some testing. I tried logging in from there (again, with a modified client) and was able to!
Is this most likely a problem with his client, or my server? The fact that I can log on from mine and log on from his on another computer makes me think that there was an issue when he downloaded something from Technic.
Thanks for any guidance.