Congrats, that is one of the two Forge RNG Bugs that randomly do not let you enter your Singleplayer World.
There is no specific Mod that causes this (well Forge causes it but good luck getting rid of that one), and it is insanely inconsistent, all you can do is try restarting MC over and over until you dont randomly crash when loading your Singleplayer World.
However there is one Mod that makes this stupid crash more likely and that is CoFH Core. I wont go too much into details, but the Bug is a Multithreading Issue and if one thing happens before the other thing is done, this crash will happen, and CoFH Core really loves to make that other thing lag hard, so that the one thing ends up crashing before the other thing is ready.