From your log, it looks like a common conflict between Mystcraft and Twilight Forest. Twilight Forest, which was just added to the pack, creates a dimension with an id of 7. If you've created six or more Mystcraft Ages, the sixth one created has taken that id. The easiest way to resolve this is to delete the Mystcraft age that's causing the problem. I'm pretty sure you just have to delete DIM_MYST7 as well as data/agedata_7.dat to take care of it. Obviously, this has a bit of a downside because it requires you to delete a Mystcraft age.
I think there's a way to change the dimension id of a Mystcraft age, which could also solve your problem by freeing up the Twilight Forest dimension ID, but I don't know whether that might affect the Descriptive Book or any Linking Books for that age. You might have to find a way to cheat in a new Descriptive book for the age. Or you could also change the Twilight Forest ID in the config, but it might have to be changed client-side as well, which would be inconvenient. So unless you really want to keep that age, removing it is probably the best solution.
