Hi @Papageno, can you please post the details in the admin screen. Click on the Copy to Clipboard button next to Server Information and then just paste it to a reply here (no further formatting required).
I Don’t have an EmonPi so this is not something I can help with directly, but I note you started on this journey last March. A revised SD card image was released in October and it may be a good time to migrate to that if you have not already done so.
I am surprised you are having these issues as this should really work out of the box @TrystanLea @glyn.hudson may be able to help in more detail.
[edit] the configuation docs of EmonHub are here.
You should not need the section above to talk to the local EmonPi as that is done via MQTT I believe rather than via the HTTP API.