A long time ago I reported this problem. Not resolved till today.
In the file /ScriptLibrary/dmxEditor/dialogs/upload.php on line 9:

<?php require_once('/ScriptLibrary/incPureUpload.php'); ?>

This cannot work, because "/" IS NOT THE DOCUMENT ROOT!

This is very easy to solve by changing the line to
<?php require_once($_SERVER['DOCUMENT_ROOT'].'/ScriptLibrary/incPureUpload.php'); ?>

But every time I edit the bahavior of the Editor or I'm adding a new Editor to my website, the upload.php will be overwritten and I have to change this line again. This is a very crappy bug and I'm sure, it's easy to solve. Why isn't yet?

I have PHP 5.3 and PureUpload 3. But the upload.php is a part of the HTMLEditor, I think.

Hello,
The generated link depends on how are:
1. Your website definition paths setup (relative to root or relative to file)
2. Link handling settings in Advanced HTML Editor 3 (relative, full or absolute)

So .. please check these.

Hello Teodor,

1. In Dreamweaver CC 2015 the path is set relative to document root. That's why the editor is in an admin area, the pages to edit have different folders deeps.
2. Link handling in HTML Editor 3 ist set to "Absolute" too.

The Link Handling has nothing to to withe the include pathes, links are only for the browser. For the browser the folder "/" is the highest directory, but for an include or require is "/" the server root. No websites exists here. When "absolue" is selected, the path for the "require" simply needs the complete server path.

I'm sure, you know that all, i'ts PHP basic knowledge.