[Koha] Plack error after update to Koha 21.11.06

[Michael Kuhn]

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
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