Hello @Christoff1,
Is it a confirmed booking? That is, it's showing in green on your calendar and you've received a notification or email saying that it's confirmed?
Could it be that the guest booked or sent a request and then sent a reservation change that is awating your acceptance?
You could message the guest for clarification. If they haven't made changes to the original request or booking then you should contact Airbnb support. Have the reservation number handy when doing so.
For info on how to contact them you can check out the document to be found as describde below. I've found them to be responsive and helpful via Twitter.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.