You might have a look at the UCR (Univention Config Registry).
root@ucs:~# ucr info apache2/startsite
Here you can configure the landing spot of your webserver. When pointing to odoo's DocumentRoot the UCS-Overview and UMC (webfrontend) is still reachable by adding 'ucs-overview/' or 'univention-management-console/' to your browsers url address.
root@ucs:~# ucr set apache2/startsite=/web
This should change the behavior as expected.