Hello Stephane,
thank you very much. The change of domain name to subdomain in the ejabberd custom-template was a good starting point.
Further I’ve needed to modify the auth script /usr/libexec/nethserver/ejabberd-auth (copying the modified version to /usr/local/… with updating the custom-template accordingly), because otherwise the users were validated against the subdomain and not the domain, which did not work
.....
use strict;
#added
use Sys::Hostname;
my ($systemName, $domainName) = split(/\./, Sys::Hostname::hostname(), 2);
####
......
my ($op, $user, $domain, $password) = split /:/,$buf;
#added
$domain = $domainName;
####
if($op eq 'auth') {........
Now it works like a charm, thank you very much!