@Lilo6 I understand that this is about a payout issue, and unfortunately you are getting the typical Airbnb robotic responses where they refuse to give you any information and tell you it's their final decision. Thousands of hosts deal with this type of attitude from them on a daily basis. They even delist hosts entirely for some "complaint" or accusation a guest made and won't even tell you what the accusation was. Like getting arrested for a crime, but the police won't tell you what crime you're being charged with. It's definitely outrageous customer service and policies.
In this case, Airbnb has done the cancellation themselves and say it is a security issue. That can happen if Airbnb has had some serious complaints about the guest (like theft, physical or verbal abuse, using Airbnbs to deal drugs or for prostitution, etc.), or the guest's credit card has been found to be invalid or stolen, so you actually might be lucky that they cancelled a guest who might have been a nightmare and a security risk to you. And if this is the case with this cancellation, then you ar beating your head against the wall to try to get them to tell you why the guest has been cancelled- they won't do it.
The reason I mentioned the reviews that were making similar complaints is that I'm thinking that maybe that's why Airbnb has refunded past guests- because they've complained directly to Airbnb and asked for a refund and Airbnb, however wrongly, has given it to them. But if that's why they are refunding, it's ridiculous that they won't tell you that, because you can't correct something if you don't know what it is.
If they keep cancelling guest's bookings who've already stayed (you can't cancel something that's already completed, though, so I don't quite understand that part) and not paying you, I'd be persistent with them. Persistence has paid off for some hosts, for others it's just been a frustrating waste of time.
Are these bookings that you haven't be paid for but the guest has stayed last-minute bookings? if so, it is probably that Airbnb didn't secure payment from the guest before they arrived and were unable to charge their credit card. If this is the case, last-minute bookings, I'd suggest you set Advance notice to at least 1 day, maybe 2. That way, you can see that the guest payment has gone through before they arrive at your door.
Very sorry this is happening to you.