WordPress: When trying to save a menu item in /wp-admin/nav-menus.php a download appears
<p>I am facing a weird problem. Unfortunately I don't find a solution online so I am asking the question here:</p>

<p>When I use WordPress and edit a menu I am trying to add, for example, a new menu item and hit "save menu".
Unfortunately, instead of saving, a download appears ("save as") that tries to save the current page (nav-menus.php) on my computer.</p>

<p>Does anyone know how to fix this? Is this a server problem?
I dont mind, if required, setting up the new menu item directly in the database.</p>

<p>Thanks in advance!</p>