Thanks again for your answer. The problem is that no new device (Jolla) was added to my profile. So I can't change any configurations. I set up a computer with the official client - that worked well.

The option "Add device" just sends me to a link where I can download various clients (I was hoping to see an option there to manually set up a device).

So it indeed seems that due to some server changes, new Prey users can't register a Sailfish device anymore.