I discovered that my upload form seems to work in all common browsers, including Firefox, Chrome and IE8.
But when it comes to IE9 it fails.
Btw, the form is in a PHP file.
This is what the file looked like:
<?
require_once ($_SERVER["DOCUMENT_ROOT"].'/_meta/phpFunctionsLibrary.php');
echo '--- HTML Code including the form element ---'
?>
Then I deleted the "require_once" line, and suddenly it worked in IE9 also.
Fortunately, in this case the Library is dispensable to a certain extent, but of course I can't rely on that.
So I need to know what could have caused this mess in IE9...