If I use this to make an external redirect, all trys to re-edit the page fail. Even if I type .../index.php?title=page&action=edit into the browser's adressbar, I'm instantly redirected to the page.
Any solutions for it?
Topic on Extension talk:ExternalRedirect
I came here wondering the same. But the extension seems unmaintained, right? Are you still using it?
The "index.php?title=Page&action=edit" URL should work (I have tested it on MediaWiki 1.37.1 with no other extensions or customizations). This is the preferred method. The easiest way is to open another page in the editor and then change page name in the URL.
Another approach would be temporary disabling ExternalRedirect (altogether or for the specific namespace/page) in LocalSettings.php.