So sorry @Chloé36 , that is so frustrating, disturbing and scary. It's unfathomable that you are basically told you've done something wrong, but will not be told what it is. Indeed, for all it's talk about transparency, trust, community and partnership, Airbnb falls very short of the mark. And as @Emiel1 alluded to, you are not alone.
https://community.withairbnb.com/t5/Hosting/Can-t-Trust-the-Trust-amp-Safety-Team/m-p/1232833
"We can’t tell you what you did wrong or how to fix it... just don’t do it again." In this case, an 'investigation' that concluded without involving the host. I don't think the host ever did find out what the 'problem' was. I hope you get this resolved in your favor soon.