NullPointerException (the error you're getting) refers to a mod requiring a file that it either cannot find or does not exist. Try redownloading the mod pack (your FTB installation is probably corrupted if there's a NullPointerException, go to Options --> Force Update, then launch) and, if that doesn't work, redownload all the mods from this location on MCForums. All the mods present on that thread are present in FTB, so better to redownload them all and ensure that things work properly.
It's interesting that only the one world is broken. Have you ever 1) crafted and placed an Ender Chest on that world, 2) crafted and placed either a Chunk Loader or a Spot Loader or 3) used Wireless Redstone? What makes this world different from other worlds, if anything? These mods are all inter-related as they are made by the same developer and share the same core libraries, so I'm thinking it's a problem with one or more of these.