Ok, so I started with a fresh Debian 11 install, using the debian 11.6 netinstall ISO provided by nerdvittles and ran through the installation.
Even the 11.6 netinstall ISO installs from the current Debian repos, and so I ended up with a 11.9 install, fully updated (note that I didn't manually run an apt update && apt full-upgrade, but right out of the installation the system is fully updated).
After waiting through the 2027D install script a while, the installation of UCP fails in the same way and at the same place as the system that has been updated with 'apt update && apt full-upgrade'
I was going to copy-paste the log, but by the time I started posting here the terminal session's scrollback had overflowed; I'll grab it later in the week. So as of today a fresh 2027D install is broken at building and installing UCP.