We recently did some changes to our mod pack which included removing and adding some mods. FML did have to make changes to our world (removing old blocks/items and injecting the new ones) . After doing this, when a user joins our server, the world starts to load and then the user immediately receives the "Server closed" message. I check the server console to discover that it's crashed with "Exception ticking world"
I went and searched around for the error, and only found one person who had the same one. However, his problem was related to updating CoFH, and we haven't done anything with any CoFH-related mods in the new modpack. One person did reply mentioned that..
There are BoP plants that require a block to remain that are all updating and popping each other. Have you removed any mods? This should only occur when a bunch of blocks have disappeared from the world
...FML had a fit and corrupted your world. You'll need to restore from a backup (if you have any) downgrade everything CoFH to the last round of releases, load the world, then update it all again in sync.