CiviMail: Can only write new mailing in debug mode (SOLVED)
Cross-origin refers to accessing resources on a different domain. It's curious that URLs you reference are actually different domains -- www.netzwerk-n.org vs netzwerk-n.org. You might want to check which of those appear in (a) the URL bar (b) the Resource URLs (esp under "?" help button) and (c) the outputted HTML documents.
As an alternative to "Debug" mode, you might try toggling disabling the "Asset Cache" mode. It's on the same screen: Administer =&gt; System Settings =&gt; Debugging, but it has fewer side-effects.