Hello,

I have a master site and some slave sites.

The domain name www.mydomain.com points to the master website.

Now I have set up a new slave website (slave x) and let www.mydomain.com point to it. That is possible of course:

www.mydomain.com and www.mydomain.com/administrator now point to the new slave site instead of the master website, which seems logical to me.

But, is there a way to still access the Master Joomla backend, when I want to use the master domain name for a slave website.

Your last sentence is not clear.

If you mean that you define a slave site with exactly the same domain than the master one then in this case, this mean that you will convert the master into a slave and that you will need another domain or subdomain to access your master.

In other words.

If your master is www.domain.com
Then if you define a slave site with domain = www.domain.com then this will convert the master in a slave.

I remember that when a URL is received, JMS Multisites starts to check amoung the list of domain defined is there is a match with a slave. If YES then this is a slave.
Otherwise (not match) this is the master (the default).