@Francis: so this is the big question, we only want to include 2 new sites that are only available without ssl. so we included the files without the ssl part. but if we include it, we get a ssl error?!
no the new files are anywhere between the other files, these files are not the first files in the alphabetically list.
at the moment i don't now if we have the word "default_server" in any file of the virtual-host files.
there are 196 files in the sites-enabled directory, maybe i have a look in the next days if there is the word "default_server" anywhere.

a few days ago we created a additionally directory for the virtual-host files and insert there the new virtual-host files.
We included the new directory in the config-file of the nginx.conf, and now it works!
We don't know why, we think that the count of the files in the "normal" sites-enabled directory are the problem or so?!
With this solution, it works correctly without any errors.
These files are the same files we had included the first the in the "normal" sites-enabled directory.

We think that the count of the 195 virtual-host files are the problem in one directory?! but we don't know it, we only believe it.