We chat about videogames and videogame accessories.

1.6.4 is automagically downloaded when you download the modpack, you don't need to worry about that.

Oddly enough, this might actually have 3 solutions
1.Update to a non beta video card driver
2.Update java
3. in a very VERY distant third for desirability
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SpoilerShow

Disable the ars magica 2 mod
(yes you may have been right at first, sorry)
If that fixes it...well you won't be able to play without us resetting the world and there has already been a significant time investment into it

but thats just an if! it could very well be the first two!
also: add me on steam so I can help in realtime

edit: tested the world save on single player without ars magica, nothing seems to be broken? this might work...