Collecting wisps to open the door worked until one of the wisps timed out. When I went back to collect it no longer follows me. This is the wisp you light the two braziers on either side of the stone slab. There are three wisps in the door mechanisms and this is the last one.
For me, leaving the necropolis and returning remedied the issue
This just happened to me as well, Well sort of. The wisp you mentioned was actually the first one I did but after doing the other three, it was back in it's spot and stuck there on the stone slap. I tried saving and reloading which glitched it even more. I ended up going back to my first save after arriving into the necropolis and redid the quest.
For me, leaving the necropolis and returning remedied the issue
