Looks like it's a problem between Optifine and Ars Magica 2. Where's the Optifine file located? This is what Ars Magica says in its FAQ:
I'm having trouble with Optifine...[/b]
AM2 supports Optifine only in the main mods directory. It will not
work if optifine is in a subdirectory. Move optifine there and it should
Note that disabling optifine by changing the extension to .disabled
currently will not work - you need to move it out of the folder
You do not need to remove Optifine. I myself have played with both Optifine and Ars Magica 2.