Internal Excepion: java.lang.IllegalStateException:Registry Entry for minecraft:blocks Block{biomesoplenty:dirt}, id 500, yield a name
I got this when trying to connect to my server
anyone know how to fix?

Sure you have the same version of BoP on both the server and client?

Yes im sure

Im using BoP with the Realistic Terrain Generation

im not sure what is wrong with the mod but i think its conflicting with one of the other mods im using
it works fine on single player

There is nothing wrong with the mod, it is something with you world. Try opening your server world up in singleplayer - if it works there, try putting it back on the server.