My friend has been having the same problem when he adds optifine to a pack. He can join and play, but it corrupts some files or something, and he ends up needing to reinstall the pack.
Trust me, Wyndman knows what he is talking about. Fixed a client crash issue for me just a day ago. Apparently FastCra(p)ft was causing my client to crash. Removed it and havent had a single crash since. Just remove optifine or get a better computer that can play the game without the need for optifine
If I had to speculate, and heck I might as well right? Optifine was never designed to actually work with modded anything, so when it gets to a new fun strange object it tries to apply all its nifty rendering techniques to the blocks, then it finds out.. YOU ARE NOT PREPARED! Then it crashes.
i'm guessing thats where fastcraft comes into play replacing optifine, Also are you aware of the permission issues with node 4 (normal mode)