Just a quick note – running Nextcloud inside an OpenMediaVault container isn’t one of the officially supported installation methods. That doesn’t necessarily mean it’s wrong, but it can lead to issues, especially with CalDAV/CardDAV syncing (e.g., missing .well-known redirects or proxy misconfigurations).
From my own experience, everything works flawlessly using the official Nextcloud AIO setup – calendar and agenda syncs reliably with DAVx⁵.
Maybe someone who uses the same OMV setup and has syncing working properly could confirm that and share their configuration?
That might help pinpoint the issue.
I’m still not sure which Agenda app you mean in detail, because a “standard” app doesn’t exist and depends on the Smartphone you are using. Many vendors are providing apps for “standard” functions on their own.
You didn’t provide a link to the app nor posted a screenshot etc.. If you are e.g. referring to the Google Calendar app with tasks support then you can’t use it because Google only allows their own calendar source. Please rephrase your description and provide IN DETAIL information about your phone, app, etc.
