[Koha] Error after upgrading to 19.05

[Jacopo]

Hello everyone!
I am running Koha on a Debian Jessie server.
I installed Koha following the "package" method and I just updated it to
the latest version (19.05).
Unfortunately, something went wrong.

What I did:
sudo apt update
sudo apt upgrade
sudo apt install koha-common #as suggested in the documentation

The latter command triggered the update of thedatabase schema. A double
check with
sudo koha-upgrade-schema
gave no error.

I then reindexed both our libraries with:
sudo koha-rebuild-zebra -v -f <library>

And restarted koha-zebra, koha-sip, koha-plack.

I am currently not able to see the frontpage nor to login in the backend
(it gives my incorrect user/password).
I checked the logs here:
/var/log/koha/koha/opac-error.log

And it gives my these errors:

404.pl: Use of uninitialized value in concatenation (.) or string at
/usr/share/koha/lib/C4/Auth.pm line 551.
opac-export.pl: Use of uninitialized value in numeric ne (!=) at
/usr/share/koha/lib/C4/Ris.pm line 833.

In particular, the error from 404.pl recurs many times.

Any help or suggestion would be much appreciated!

Thanks!

Jacopo