Synchronisation with Nextcloud instance in private network not possible even when connected via VPN

[hapst3r]

Describe the bug
Synchronisation does not seem to work when the Nextcloud instance is hosted in a private network, even when I am remotely connected by means of a VPN.

To Reproduce
Steps to reproduce the behavior:

1. Nextcloud instance is private
2. Add Nextcloud integration account, no tasks or projects are imported, it is impossible to create tasks or projects
3. See that CalDAV synchronisation works with others programs such as Thunderbird (same account, same credentials, same link to resources)

Expected behavior
Functioning synchronisation when within private network, not working when outside of private network (not connected via VPN).

Desktop (please complete the following information):

• openSUSE Tumbleweed with GNOME (both up to date), using the Network Manager VPN applet which is integrated with GNOME
• Planify 4.11.4, installed via flatpak

Additional context
I have not found any logs which I would have liked to check out to be more helpful when reporting the bug. I am happy to provide more information as needed!

Thank you for making this program, though <3 :)