I tried your media file URL with my own test site and it verified successfully. Most likely, there's a problem with the server hosting your website. Contact support for your web hosting company. They can check their logs and see if they can track down the source of the issue.
Note that even if you can't verify the media file URL within PowerPress, you can still publish your Podcast Episodes. Just be sure to manually enter the episode duration and file size then publish the post as you always have.