I think I found the problem/solution.
For safety reasons I made a copy of "ocsinventory-reports.conf" in the /etc/apache2/conf-enable directory and called it "ocsinventory-reports-org.conf".
I think, for some reason, php took the configuration of the "ocsinventory-reports-org.conf" instead of "ocsinventory-reports.conf".
I removed the copy from the etc/apache2/conf-enable directory, restarted apache2 and everything worked perfect.