I have the same problem, but i killed Alexander at the elven temple and brought his head to the Sallow Man (who i killed as well afterwards).
When i interact with the wellspring the interacting character gets zapped and Dallis and her cloaked figure appear, i get a few seconds of camera movement but no dialogue voice or otherwise whatsoever.
After the camera movement the game is back to normal, interacting with the wellspring does nothing and Dallis just stands around.
