It's not really so much an error, more of a small piece of information that's been neglected.
Arinth the Mad cant be broken out of the cell without someone using his staff, that bit is clear. The bit that's missed is that you need a level 28 combat mage in order to handle the staff.
I never trained any of my characters up as combat mages ( just nature up to a point where they can autocast the low level heal spell, except Taar ) and coming back before I finish the game to get all the secondary quests I discovered I have no one to carry out the simple task of taking a swing at the cell. I'm currently running round with Finala trying to get her up to 28 ( she started level 21 or 22 ). A bit of a pain in the butt having to run round using her when she's just got base items on so I can set her quests off.