Setting up sub domain doesn't work

I tried to set up (domains -> add domain) but always get the message:

“Another user owns If you’d like to manage this domain you will need to log in to the web panel as this user”

I know this worked before so it must be a bug.

Does it mean that it is not possible to create any more?

No, they put an end to that so they could reserve it for new servers. However, you can now create a free subdomain.