After hours and hours of debugging and searching I found the reason and the solution.
Situation:
Domain with hosting created, after reload of http website won't displayed (only the default website from plesk)
Reason:
In Tools > IP Addresses 2 or more IPs were added, but the main IP had an other Netmask.
Moving the page to the first one (internal ip) allowed me to access the page internally.
Solution:
You have to edit the IP(s) or to delete and recreate them so that all IP(s) have the same Netmask.
After that, all were able to access the websites.
If you can't change the IP, you can delete it via mysql (experimental):
mysql -uadmin -p`cat /etc/psa/.psa.shadow`
use psa;
select * from IP_Addresses WHERE ip_address = '<YOUR IP>';
#Write down the ID
DELETE FROM IP_Addresses WHERE id = '<ID>';
DELETE FROM ip_pool WHERE ip_address_id = '<ID>';
Afterwards you can create the IP again with the correct Netmask.
Be sure, that you have moved all connected domains to another IP!