Having trouble to run UrBackup in Docker as Server & Client

I just stumbled upon this great tool. I managed to host a server over my docker and successfully added my windows pc as a client. I didn’t manage to add my raspberry pi running the server in docker though. Onlinestatus is “no” and when I try to do a full image backup it (obviously) says: “Never.
Starting backup failed”.
Is it even possible to have a server and a client run within 1 system?
Thank you and a happy new year :-)!