When accessing the webmail part of Open Xchange access is very slow. For example, opening the inbox already takes about 20 seconds.
You might check the file
/var/log/univention/listener.log or other files for entries.
uid=chris,cn=users,dc=multi,dc=ucs "['/opt/open-xchange/sbin/createuser', '--adminuser=oxadmin', '--contextid=10', '--username=chris', '--email=chris user email@example.com'
Note the wrong email address which contains invalid spaces. Therefore the listener fails and needs to restart frequently which causes delays.
Make sure all users are created with correct attributes.