We nailed this issue down. Discovered it has to do with the 'auto-level up' feature with the base game. We mistakenly added px2 abilities to certain characters progression tables when they auto-level up that they do not have access to. If you disable this feature in the options menu you can bypass the bug. We are however trying to get a fix, though I have no exact date when it will release.
Sorry
-Sking
I was having the same problem (with Hivarius). This solution worked, but only after RESTARTING the game after disabling the 'auto-level up' feature, and loading my last save before having Hivarius join my party.
Game breaking Bug - Cannot change Party (3.03)
in Pillars of Eternity: Technical Support (Spoiler Warning!)
Posted
I was having the same problem (with Hivarius). This solution worked, but only after RESTARTING the game after disabling the 'auto-level up' feature, and loading my last save before having Hivarius join my party.