Moving to a new server gives me head ache

I have a multisite (subdirecory) installed and runnig for one year.
Now we hit resource limits and moved to a cloud computer.

I moved the entire directory structure and the database and adjusted the database data. The physical directory is to /var/www/abc.com
Since the blog is still running, I added the new IP to my local host file, so that a http://abc.com points to the new IP address.

I get only the file http://abc.com/index.html displayed.
I used to go to the main blog with http://abc.com/wp-admin but that says, does not exist.

I cannot figure out what I could have forgotten. Can anybody give me a hint?