Visited today and could not charge because all the spots were ICEd.
The 6 charging spots are the first to the right on level P1 after going down the ramp.
The parking was full and there was a bit of wait to enter. The "free" sign above the entrance never changed though. Maybe because it doesn't work, or maybe it only says "besetzt" if people stop coming out for a given amount of time.
I wonder whether they re-configured the parking spot count after adding the charger. There were probably ~20 parking spots announced free while cars were driving around trying to find them, so the cars ICE-ing the chargers could probably have found other spots. The total allowed cars in the parking should have been reduced by 8 as they removed 8 regular parking spots to create 6 charging spots (they are a bit larger than regular spots).
When I returned back an hour later the parking and charger spots were still full but 2 actual electric cars were able to catch a spot in and charge.