Hi We were updating from Debian GNU/Linux 9 to 10, then from Koha 20.11.03 to Koha 21.11.06. This is a Debian package installation. Now Plack throws many error messages like the following in file "/var/log/koha/plack-error.log": Error while loading /etc/koha/plack.psgi: Can't load application from file "/usr/share/koha/api/v1/app.pl": Can't locate JSON/Validator/OpenAPI/Mojolicious.pm in @INC (you may need to install the JSON::Validator::OpenAPI::Mojolicious module) (@INC contains: /usr/share/koha/lib /usr/share/koha/installer /usr/share/koha/lib/installer /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.28.1 /usr/local/share/perl/5.28.1 /usr/lib/x86_64-linux-gnu/perl5/5.28 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.28 /usr/share/perl/5.28 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base /var/lib/koha/whu/plugins) at /usr/share/koha/lib/Koha/REST/V1.pm line 25. BEGIN failed--compilation aborted at /usr/share/koha/lib/Koha/REST/V1.pm line 25. Compilation failed in require at (eval 507) line 1. Compilation failed in require at (eval 506) line 1. Child process 21538 exited with status 255 I see there is bug https://bugs.koha-community.org/bugzilla3/show_bug.cgi concerning this problem but there doesn't seem to be a solution for our case. On our Debian GNU/Linux 10 system package "libjson-validator-perl" is the newest version (3.06+dfsg-1). Does anyone know how to solve this problem? Best wishes: Michael -- Geschäftsführer · Diplombibliothekar BBS, Informatiker eidg. Fachausweis Admin Kuhn GmbH · Pappelstrasse 20 · 4123 Allschwil · Schweiz T 0041 (0)61 261 55 61 · E mik@adminkuhn.ch · W www.adminkuhn.ch