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 🙂