I cannot think of a reason why installing both apps should block login attempts, especially if you haven't configured either of them yet.
First, please verify that the time & date inside the UCS VM is correct.
Next try changing the password of the "administrator" user via the console. You can do that with the following command:
udm users/user modify --dn uid=administrator,cn=users,$(ucr get ldap/base) --set password=YourNewPassword
A successful change should output something like this:
Object modified: uid=administrator,cn=users,dc=…
Now try logging in at the Univention Management Console with "administrator" and the newly-set pasword.