The error means that somehow you had an ongoing fucking action with either zero givers or zero takers. Unlikely to be the result of using a save from 0.5.24. More likely that either a check was missed or a bad assumption was made somewhere in the code. Without knowing a specific set of actions during the interaction that triggers it, there is little that can be done.
Slaves all behave roughly similar once captured, though some NPCs have quests that continue in your mansion. The acquirable NPCs are pretty much the rare slaves of the game, as they have unique traits and pictures.