Thank you for your reply.
No, I have not entered any domain transfer key. With the transfer, I got to the point where inside the Cloudflare admin panel, I was asked to enter the transfer key. I had requested the key from the previous registrar. But it took too long to arrive in my inbox, so I logged out of Cloudflare thinking I could enter the key anytime later. When I had received the key the next day, I logged back in to Cloudflare – but there is no place to enter the transfer key now.
The domain I want to transfer still shows “Transfer in progress” with a link to “Zone overview”. So the transfer, I assume, has not been completed yet.
Do you know how I can submit the transfer key to complete the process?