AHA! YES! I am not the only one. I also have this problem AFTER loading a world or joining a server, trying to go back to singleplayer world and load it up it will sit there for a moment go back to main menu, you try and do it again? it locks up and you have to end the process. I suspect it may have something to do with ram as I have 211 mods loaded and have only allocated about 3-4 gigs on my client itself. What modpack are you using and how many mods are loaded and how much ram do you have allocated to your client?