As far as I know, I had this .htaccess file in the installation package, which I downloaded. Also do not know why the rewrite rules looked like this.
I now have uploaded your hand recommended .htaccess file and still receive error messages such as these:
POST http://flarum.wpzweinull.ch/api/tags 401 (Unauthorized)
They always say that the resource can not be loaded.
Probably I still have something wrong in the installation, but just what exactly ?!
The .htaccess file should be good now.