Thank you everyone for the replies.
I guess then it's over and out for me as far as Airbnb-hosting goes, while living in this place. Next time I buy a property I'll have a deeper look into the lease contract beforehand...
It's a pity really... we enjoyed hosting not only because of the (rather small) extra income but also to meet people and enable them to make their travels more affordable (since we love travelling ourselves).
In my opinion renting out a whole property vs a spare room on Airbnb is different. I think there has been a whole flat rented out on Airbnb in our building and that there may have been noisy parties etc. in it which then attracted the attention to Airbnb.
However, renting out our spare room is very different from that, because we have been very careful whom we accept to host as we have to share the flat with our guests. I'd say for me this is not much different than hosting friends or family. Unfortunately it's not treated the same way in the lease contract.
Living in a city where the housing shortage has been described as critical, you'd think preventing sub-leasing a spare room for a longer term should be made illegal! Also for many in this ridiculously expensive city it may really help some to pay the bills if they can get an extra income.