What it says on the tin. This isn't exclusive to one specific Flatpak btw, I can confirm this is happening on both Kdenlive and Vivaldi.
Vivaldi in particular claims the download was cancelled the moment I click Save. Kdenlive will render files just fine, but it won't save project files. It doesn't even respond to mouse input, but it will respond to keyboard input (i.e. any key combination, like
Alt+F
Alt+F
), and it'll only take mouse input if something in the UI is refreshed (a file gets imported or something like that)
I've checked Flatseal, and both have access to the home directory, which is where I choose to save these files.
Solution
Don't necessecarily know what your issue is, but xdg-document-portal is a systemd-user service. You need