I recently set up a server running just a few mods for me and a few friends. I triple checked every mod was compatible however when I log in I get an odd error, my client crashes giving me the screen posted below. Server side it just says I don't have FML installed, and cannot join.
Mods Installed (In order of installation on client):
Forge Universal
Too many items
Rei Minimap
Mine Little Pony
Player API universal
Smart Moving
In the client side mods folder:
Computer Craft
Custom NPC's
Server mods installed:
Forge (running this from the universal forge.jar)
Player API
Mine little pony has one conflicting file with Forge, inserting it into the mods folder or forge on the server did not solve the issue and was removed once I got the same crash.
Let me know if there is any more information you need.