[Koha] Upgrade to 24.11.01 blocked in maintenance mode

Hi, I have a small private instance of Koha, on a Debian server, installed with the Debian packages. After the upgrade to 24.22.01 the web front sends me to the installer, and the OPAC says the site is in maintenance mode. If I log in to the web installer, which says "Web installer › Updating database structure" but after the first button, I just get a "Try again" button in eternity. I have run from the command line as root, with the koha-common service stopped, both "koha-upgrade-schema XXX" and "koha-rebuild-zebra -v -f XXX", without any errors, but I'm still sent to the installer on the web end. I can find nothing relevant in the log files. I've rebooted the server, restarted the service etc etc. I'm stymied. -- René