Nextcloud - be able to work offline

[cyb3rm00n]

Is your feature request related to a problem? Please describe.
I have a nextcloud server in my house that is not exposed to the Internet. When I use Planify outside the Nextcloud syncs are blocked because I get an HTTP error (I guess because it can't connect to my server). This specifically happens when I want to "tick" the task, it is possible however to add a description to the task without any error.

Describe the solution you'd like
The possibility to be able to fully work offline (so it's possible to tick a task) and then sync the changes once there is connection with the Nextcloud server.

Additional context
Error I get when trying to "tick" a task:

[alainm23]

Yes, it is a task that I still have pending