Just tried this on a fresh installation but there I was able to add a comment:

image.png

Have you installed any plugins which may interfere here? Are there any errors on the app server side or in the browser console?

Thanks for the hint here, I've fixed up the meta info for the next package release.

Thank you both @murgero @nebulon
I applied this in the Kanboard app security's tab in Cloudron:

frame-ancestors https://link-of-the-target-website.com;

Now it works! 😄

@girish

Yeah that is the project for the integration, it also needs the Signal CLI
https://github.com/AsamK/signal-cli

Brilliant, thanks @girish

Apparently, it was related to email notifications.

@girish Many thanks for that! That did the trick 🙂