Question is, where was the backup taken? On which system?
As mentioned, the backup was taken from a non-dietpi system. It ran on a Raspberry Pi 3, and Octoprint was installed using GitHub - guysoft/OctoPi: Scripts to build OctoPi, a Raspberry PI distro for controlling 3D printers over the web , the recommended installation option for Raspberry Pi’s.
Does the original system, that has been backed up, does it had port 5001 set? Yes or no?
No. Octopi apparently uses Octoprints default port (5000). However, you never had to explicitly specify a port number, it was abstracted away by OctoPi and you could just navigate to octopi.local (without port number).
If yes, it would be a question to app developer why the port was not saved.
It could still be… If they would have explicitly saved the port number 5000 to the configuration file, I would have found it there. Now it was implicitly using the default value.