Hi thommie,
I think you will have to set your own Content Security Policy permitting the domain netzwissen.de.
When I read the gist MyCSP app for ownCloud · GitHub correctly (coming from How to correctly set CSP?) you would have to create an app, where you use addAllowedFrameDomain instead of addAllowedScriptDomainin the example.
All available methods are found at core/EmptyContentSecurityPolicy.php at master · owncloud/core · GitHub
I have not tried it myself, so let us know, if it is working 