I reloaded an older save, and now it went according to script: after I wasted Okku, a dialog ensued, there was a short exchange between them and Kaelyn, and off they went. No other major weirdness then.

Some minor weirdness, though: occasionally summoned critters which are supposed to be unsummoned on map transition linger on as "ghosts" — their character icons stay on but with the health bar at zero. Not even summoning replacements exorcises them. Saving and reloading will, though.