Double slashes in image URL after browsing server

I am using CKEditor 4.2 with KCFinder 2.5.1. After calling the image dialog and browsing the server the URL of the selected image starts with double slashes (//).

What have I to do to fix that problem? Please advise me.
Thank you.

Extract of my CKEditor config.js:

...
config.filebrowserBrowseUrl = '/kcfinder/browse.php?type=files';
config.filebrowserImageBrowseUrl = '/kcfinder/browse.php?type=images';
config.filebrowserFlashBrowseUrl = '/kcfinder/browse.php?type=flash';
...

Extract of my KCFinder config.php:

...
'uploadURL' => "/",
'uploadDir' => "",
'thumbsDir' => "/kcfinder/thumbs",
...