@Tiziana57
Since you were quoted 'data protection' it is impossible to enter into the discussion with anything
other than conjecture or speculation.
It would only be more helpful if you had translated "unrechtmässig" as other than 'I suppose some kind of irregularities'. I'm not a German speaker, but an online translation gives 'UNLAWFUL' or 'ILLEGAL'.
Moreover, the fact that it was dealt with by humans at Airbnb serves to differentiate it from our common experience of 'perfectly innocent content erased by the system as "hidden content",' which is clearly computer generated - also human programming at source, but probably not checked at every turn by Airbnb Safety & Security.
Consequently, it seems you are left to chase your tail in a frustrated quest for an explanation that you are unlikely to ever receive. And you are probably still at it because of the singular reason of it being 'low season' and potentially losing income. In other words, if the booking was quickly replaced, you probably wouldn't spend so much time preoccupied with this matter.
The best you can probably hope for if other Hosts on this forum inform of similar cases, which is clearly what you would like. So lets see if anyone else contributes.