It is normal that you have an error message on the
www.familie-spiegel.de/testing-jms/multi...onfig_multisites.php because this is a JMS configuration files that can not be directly access from internet.
The URL
www.familie-spiegel.de/testing-jms/multisites/dummy is working perfectly and I don't receive any 403 error.
The fact that
www.familie-spiegel.de/testing-jms/multi...onfig_multisites.php return a blank page is also correct because this is again an internal configuration file.
The access to
www.familie-spiegel.de/testing-jms/multi...my/configuration.php may return 403 and this is not a problem as you can access your website with the index.php that is the normal entry to your website.
It is normal that if you try to "hack" your system that you don't retreive it info.
So till there, I don't see any errors.
The website is correctly displayed in the
www.familie-spiegel.de/testing-jms/multisites/dummy and I see a dummy menu.
The hello world is also correctly displayed.
So where are the errors that does not allow to access your slave sites. If the error corresponds to your attempt to hack the joomla files and hack the JMS, this is normal that you can not display them. They are protected by security.