I have had some problems with broken downloads with server 503 messages which is a fatal problem for something like Flatpak downloading program files. I have tried a few times this week to install mypaint and kicad using Flatpak, mypaint for the later release than Debian has and to sandbox kicad, breaks your updates. I take it that this isn’t Flathubs problem, but the developers sites.
So the trick appears to be try again later, use another time of the the day ,or night and you will get what I had this afternoon “Sweet Success!” Two things I would like to clarify does Flathub have someway of notifying them and if you hit this problem it is generally temporary so try again later.
Thanks for a great service!!!
The issue is tracked at https://github.com/flathub/flathub/issues/845 and https://github.com/ostreedev/ostree/issues/2022.