When placing a Command Block Minecart and pushing it on powered activator rails within ANY command, all players will be kicked with the reason:

Internal exception: java.io.IOException: Received string length longer than maximum allowed (88 > 64)

and will be unable to connect to the server (every time it will kick with the same reason). Doesn't happen in Singleplayer.
The best way to fix the problem is to download the world, open it with MCEdit and delete the minecart with command block.
I've also tried this command in command block minecart:

/summon FireworksRocketEntity 95 68 -156 {LifeTime:0,FireworksItem:{id:401,Count:1,tag:{Fireworks:{Explosions:[{Flicker:1,Trail:1,Type:1,Colors:[40000],FadeColors:[0]}]}}}}

and it showed a similiar error.