So from what I see, you have 2 issues:
Firstly:
- index.php is not being found. This is generally because of a) an upload error, or b) missing directory traversals within the script (../../ etc..) What we need to know is why index.php is not found, and why it is being called when you try to make a change
Secondly:
- The link generated when the email is sent, is incorrect. This will just be a case of finding where the link is generated, and modifying it to generate a correct code.
If you could update the post with source for the files, then I may be able to help you further.
If you feel you can't do the above, then I suggest using a different, more mature (and better coded), mailing script for your site.