Screenshot from 2022-09-26 19-36-11.png Edit (149.8 KiB, image/png)

@Simon Here goes, although now it just shows both at 9.7, and 9.9 not yet available. Before I downgraded libc6 it obviously showed libc6 at 9.9.

I also tried to reproduce the issue in a fresh VM, but those kept getting 9.9 for all the packages, so installing build-essential caused no issues.

I don’t have in-depth knowledge about phased updates, but this looked like as if downloading the updates during installation was not yet affected by phased updates (and hence got libc 9.9), but, after the installation was completed, the system got cast into a ”still at 9.7” group of phased updates, and so libc6-dev (et al) could not be installed.