@BrutalBirdie While playing around to overcome this error. I found an alternative solution. It doesn't fix the collabora CODE font issue mentioned here, but makes it possible to add fonts to a Nextcloud instance hosted via cloudron.

It works with Nextclouds' ability to add fonts as remote_fonts therefore do the following:

Add the desired fonts at https://[YOUR.NEXTCLOUD.DOMAIN]/settings/admin/richdocuments
Switch to the apps-filemanager of your cloudrons Nextcloud. Create the folder ~/apps/richdocuments/settings
Find out in which [APP-DATA-DIRECTORY] Nextcloud saves the richdocuments-data inside the directory ~/data (e.g. appdata_ocv42l2jdx7f)
in the newly created folder, create a file fonts.json (so ~/apps/richdocuments/settings/fonts.json exists) with the content

{
    "kind": "fontconfiguration",
    "server": "[YOUR.NEXTCLOUD.DOMAIN]",
    "fonts": [
        {
            "uri": "https://[YOUR.NEXTCLOUD.DOMAIN]/[APP-DATA-DIRECTORY]/richdocuments/fonts/[FONTNAME-1].ttf"
        },
        {
            "uri": "https://[YOUR.NEXTCLOUD.DOMAIN]/[APP-DATA-DIRECTORY]/richdocuments/fonts/[FONTNAME-2].ttf"
        }
    ]
}

restart the collabora app
restart the Nextcloud app

Additional, remove any font added to the collabora fonts directory...

Edit: After trial and erroring even more, I found nextcloud updating the fontlist only if ~/apps/richdocuments/settings/fonts.json is again saved.

Edit-2: Unfortunatly after a collabora update nextcloud needs to be restarted again to make the font changes again available…

@nebulon maybe this might solve the issue?
cheers

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