You can rename the constants.php file to something else to test. I can take a look at why it is looking for a dev folder if you are able to provide FTP access:
We would need to look further into this for you, however the access details on file appear to be incorrect or missing. Could you please update these details by visiting your client area, selecting the relevant purchase, then clicking "Review/Update Access Information" under the "Stored Access Information" section.
We look forward to further assisting you.