@Rebekka-and-Sandie0 First of all we need to clarify whether the communication in January was an inquiry or a reservation request. The reason I ask is because you cannot send a Pre-Approval as a response to a reservation request. Pre-Approvals are optional responses to inquiries only.
Assuming that it was a reservation request, you are supposed to respond within 24 hours by accepting or declining the request. Sometimes guests ask questions in a reservation request, but just answering does not satisfy the response requirement. If a reservation request is not declined or accepted within 24 hours, Airbnb is now blocking the dates of the request. Read this help article and scroll down to the bottom where they talk about Expired Reservation Requests
https://www.airbnb.com/help/article/28/what-do-i-do-after-i-receive-a-reservation-request:
Hopefully I've interpreted your situation correctly. If not, please give me more details!