OnlyOffice is fully functional with OwnCloud but not with OwnCloud Web

[cromanpa94]

Describe the bug

A clear and concise description of what the bug is.
OwnCloud's OnlyOffice integration is fully functional. However, the integration with Owncloud Web has issues (i.e. even though OnlyOffice is recognized by OwnClowd web).

Steps to reproduce

Open any document in OwnCloud Web using OnlyOffice (please see details on my set up below).

Expected behavior

Documents should open in OnlyOffice through OwnCloud Web. Note that OwnCloud is correctly configured with OnlyOffice (and I'm pointing (I think) at the same .js file from the .json (see below as well).

Actual behavior

OnlyOffice works with OwnCloud but not with OwnCloud web.

Setup

Please describe how you started the server and provide a list of relevant environment variables or configuration files.

1. OwnCloud deployed using Docker (portainer; synology; reverse proxy enabled)
2. OwnCloud web installed following https://owncloud.dev/clients/web/deployments/oc10-app/. Note that I'm using the version from the app.
3. OnlyOffice deployed using Docker (portainer; synology; reverse proxy enabled). Fully functional with OwnCloud
4. OwnCloud's config.json updated and onlyoffice is available for OwnCloud web as shown below:

5. Opening the document in OwnCloud Web results either in this message:

Or this other...

This is an extract of .json file:

  "external_apps": [
    {
        "id": "onlyoffice",
        "path": "https://XX/index.php/apps/onlyoffice/js/onlyoffice.js"
    }

Here's the structure of my config folder:

And the OnlyOffice files within OwnCloud:

Additional context

Any insights and pointers are appreciated.