For those internet folks who found this post, but not the answer...
Some mods overwrite the vanilla minecraft blockids. Those mods do this when you start your client, even before you connect to the server. The server then checks your IDs, and sees you have a conflict, but its with a mod that the server does not have.
So on your client, you have a mod that the server does not have -- and this mod has overwritten a vanilla minecraft blockid.
The solution is to remove that 'extra' mod from your client. Coping over the server configs will not help you, because the mod is not on the server.