Thank you for the informative tip, Manos. For other readers, the summary is that the problem may be caused by the installation of an unacceptable version of a particular Perl module, JSON::Validator. What I find on our own server is the following: # sudo cpan -D JSON::Validator JSON::Validator ------------------------------------------------------------------------- (no description) J/JH/JHTHORSEN/JSON-Validator-5.04.tar.gz /usr/share/perl5/JSON/Validator.pm Installed: 5.02 CPAN: 5.04 Not up to date Jan Henning Thorsen (JHTHORSEN) jhthorsen@cpan.org # apt-cache policy libjson-validator-perl libjson-validator-perl: Installed: 5.02-1 Candidate: 5.02-1 Version table: *** 5.02-1 100 100 /var/lib/dpkg/status 4.99.1+really3.14-koha3 500 500 http://debian.koha-community.org/koha oldoldstable/main amd64 Packages 3.06+dfsg-1 500 500 http://mirrors.digitalocean.com/debian buster/main amd64 Packages Note the version contained in the Koha repository. If the version of this module is indeed the problem, I have to admit that I'm not sure how to go about forcing the installation of the correct one—Manos' solution in the GUI's package manager won't work in this server environment. It's a production instance, so I have to tread lightly. Again, my thanks go out to all readers and contributors!