Web interface does not show uploaded files not contact

Problem statement:
Web interface on CENTOS 7 laptop does not show uploaded files nor contacts that were loaded. I used three different browser (firefox, Chrome, Chromium), all show the same result
Next to all icons on the status bar I see something that looks like a circling progress indicator. But nothing else happens.

My setup is the following:
Server: CENTOS 7.5 , php-7.2, httpd-2.4, MariaDB-server-10.1
Clients: WINDOWS 10 client, iPad, iPhone, CENTOS 7 laptop

The server is up and running. The clients are installed and connect/sync fine.
When logging in from WINDOWS 10 (same LAN as CENTOS 7 laptop) the web interface works just fine.
There are no error messages nor warning in nextcloud.log nor in apache related error logs.

Thank you for

Just to add, I found these error messages in firefox and chrome… Any idea what these mean?
This only happens on my CENTOS laptop. On my WINDOWS 10 in Chrome, it just works fine.

In case somebody else runs into the same problem, I had to disable Content Security Policy (CSP) in firefox browser to make this problem go away.
1 - about:config
2 - security.csp.enable - fals

Is somebody aware of another solution?