Luckily, I had a savegame just before dealing with that area, and could finally find out what really happened here. Your problem most likely is that Ponikenus has seen you while you were approaching. If he sees Elizabeth, he will try to cast a spell at some blue sphere which is mounted on some kind of upside-turned-down stone claw inside the building there. This spell enlarges a holy aura around the sphere so much that it extends outside the building. You need to run through that aura and get to the other side of the building, where Ponikenus once was - or load an earlier savegame and try another approach at Ponikenus and his guards.

If you cannot survive running past the building because of Elizabeth's damaged health, you have to return to one of the white "things" which heal Elizabeth. One such thing is in the castle's kitchen, another in the armoury below the castle. A third one is close to where you are now. Open the gate near the bottom of the ramp with Thurzo's keys, then go through the passage behind the gate. It leads to the mill, but somewhere there, you should find one of the white "things" which can heal Elizabeth.

Once you are strong enough to get to the other side of the building at the top of the ramp, you have to fire one of Elizabeth's fire spells at the blue sphere in the building. That should destroy the aura here. Go back to your preferred healing place if you like and come back. Approach the chest behind the altar and destroy the holy water flask with a spell, then take the key out of the chest. Use it to open the hatch in the building and proceed to Katalin. Beware of the next aura...