Looks like Flatpak manifests in distributed nightly and Kube source code are different. In particular, nightly seems to not even use KDE runtime/sdk, but builds its own qt/kde stack. Not sure why is that necessary.

In Kube repository, Flatpak manifest is pointing to KDE/5.12. According to your blog, you moved to KDE/5.14. Could that be updated?

Finally, Kube repo is downloaded from git://anongit.kde.org/kube.git . For me, anongit asks to use https, not git:// leading to the failure in the download:

fatal: remote error: Please use the https: protocol to connect to anongit