[Koha] Branch library list empty

[David Liddle]

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 at 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!