[Plugin: Link Library] User link submission error

Our site has user submission forms for a few link categories, all of which are returning the error below; any suggestions as to where to start troubleshooting?:

The website encountered an error while retrieving https://www.safemilk.org/wp-content/plugins/link-library/usersubmission.php. It may be down for maintenance or configured incorrectly.

Thanks in advance for any help.

https://www.ads-software.com/extend/plugins/link-library/