I've had the same problem. Just putting it in the 'authorization' list doesn't work I'm afraid :/. You have to open up the configure tab, then go to the 'on-access scanning', then add the nexon/mabinogi files to the exclusion tab(I just added the folder). Do the same with the on-demand extensions and exclusions. It won't delete the game anymore(so you won't have to reinstall it, but make sure you install it with sophos off first). However, from the one time I started the game, it deleted some temp files mabinogi made, which I don't expect is a safe thing to have happening. As such, until sophos decides to stop finding mabinogi as a virus, you might just have to turn off sophos whenever you play the game.
This actually happened previously about a year ago, but I didn't do anything at that time(let it stay deleted) because free rebirth hadn't come about yet. Sophos eventually got its act together and stopped calling it a virus sometime later(since I was able to successfully reinstall the game sometime after Iria). Not sure how long it took though(since I sorta ignored the game for a while).
