To make the authentication against PAM actually work one of the systemd
service files needs to altered. The c2s part of jabberd needs to run as
root to be able to read /etc/shadow. One way to accomplish this is to
create a jabberd-c2s.service file in /etc/systemd/system with
following content:
.include /usr/lib/systemd/system/jabberd-c2s.service
[Service]
User=root