Embarassing, already (partially) solved. I didn’t spot requests made to the OnlyOffice server and was assuming it failed before trying. But enabling wopi-support (and allowing the additional domain) in OnlyOffice already fixed the problem – except that it still just downloads files instead of opening the editor.
I have noticed that app-registry.yaml and tried to add it into the config directory as well as pasting it in the main config. It’s either not recognized or something else is wrong. I have not spawned an additional ocis appservice due to conflicting ports.
In the ocis.yaml i of course also adjusted each entry from the Docker example. Such as
- mime_type: application/vnd.openxmlformats-officedocument.wordprocessingml.document
extension: docx
name: Microsoft Word
description: Microsoft Word document
icon: ''
default_app: OnlyOffice
allow_creation: true