The problem was solved by a fresh installation, using a subdomain (!!) instead a folder during the installation process.
>> I used 'ossn.mydomain.tld' instead of 'mydomain.tld/ossn'
Allthough ossn does not require this, it has helped.
The issue didn't appear before and didn't generate any error logs. The html of the 'message' and 'notification' site were thrown out, but not displayed. The only reason this could have, was a file that got damaged while uploading.