To change Confixx's hostname (i.e. reaching the login under a different / new domain) is quite complicated.
Proceed as follows:
1. Log in to Confixx as administrator
2. In the left menu click on "Einstellungen" -> "Standard-Domain"
3. Enter the new domain here and confirm your input
4. Conduct the Confixx update script.
Confixx should then accept the new domain as standard domain.
Versions 3.1 and above do save and display the new standard domain in the database but it doesn't work.
Reason: It's not entered as ServerAlias in the confixx_mhost.conf file.
Corrective: Edit the file /etc/apache2/confixx_mhost.conf and paste the following line under the "ServerName" line in the <VirtualHost xxx.xxx.xxx.xxx:443> area: ServerAlias the_standard_domain.tld, where standard_domain.tld has to be replaced with your domain.
This solution may not be very smart since the mhost file could be probably overwritten by Confixx anytime, but it provides a remedy until the error is fixed.