When we arrived at the apartment the key wasn't in the box, where it should have been. We could not get in, and we couldn't reach anyone and neither could Airbnb Support. So we were on the streets and after waiting for a response for some time we had to find another accomodation at 22:00 (which is not easy in Venice on a long weekend). The room we found for that night was way more expensive and much lower standard (1 bedroom for 5 people sharing 1 bathroom with 2 additional guests). The next day we were able to check in (cleaning staff had forgotten to put the key in the box) and the apartment itself is fine. We demanded a refund for that night but they are only going to refund the price for the apartment for one night, which is a lower price than we actually payed for the room we had to book spontaneously.
All in all, we had to go through a lot of stress and trouble on our weekend vacation and additionally have much higher expenses for our accomodation than planned.