Turned out that the recent Flatpak package (0.8.5-1) was missing on the mirror I use. I have installed it manually and it looks like that the issue was fixed.
** Changed in: flatpak (Ubuntu) Status: New => Fix Released -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1657912 Title: Flatpak hangs when trying to install org.gnome.Platform runtime To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/flatpak/+bug/1657912/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs