Meanwhile I found out, there is no management agent installed on the servers.
I downloaded the Server Security package from ESET webpage and entered my credentials during the installation. It is activated with our company's license, but no agent is installed. This might be reason that the servers aren't shown.
So, now I want to install the agent and downloaded in PROTECT Cloud via Quick Links the msi Package for Windows devices and started installation on my windows server. During installation I'm asked for Network connection to ESET PROTECT Server. The default values are localhost:2222. What do I have to enter when I use PROTECT Cloud only and no on premise installation of ESET PROTECT?