This file opening problem again caused wallace to crash on my server, resulting in the failure to deliver emails to the end recipient.
Searching though the wallace directory I found the following instances

Same file opening problem exists in /usr/lib/python3.9/site-packages/wallace/module_invitationpolicy.py - line 270 :-

Also applies to EL9.
One solution appears to be to change the declaration of $ready , in class/kolab_ldap (files /usr/share/roundcubemail/plugins/libkolab/lib/kolab_ldap.php and /usr/share/roundcubemail/public_html/assets/plugins/libkolab/lib/kolab_ldap.php) from private to public.
There is also another problem, the directory /usr/share/roundcubemail/program/resources containing the "Ooops ..." error message that, presumably, should be displayed on crashes is missing.