So it seems like the cause of the problem was that the package was built against qt 5.13.1 in staging (one of my colleagues did a rebuild for mathjax), and mnju was running with qt 5.13.0 in stable (since 5.13.1 is still in testing).
Yay to more fragile ABI mismatches. Also no wonder I could not duplicate the issue, since I always run against [testing]. Thanks all for bringing it to our attention, and, we've rolled out a stable rebuild and are in the process of rebuilding for testing too.
Everything should be working momentarily.
...
Incidentally, the error message as shown by ebook-convert involved:
Code:
qt.qpa.plugin: Could not find the Qt platform plugin "headless" in "/usr/lib/calibre/calibre/plugins"
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
Available platform plugins are: headless (from /usr/lib/calibre/calibre/plugins), eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.
Aborted (core dumped)