Since this is a "not found" error you will want to make sure that the file exists on your account, is being referenced correctly, and has the correct file permissions before you proceed.
This common error can occur in several site applications that send funny looking URLs to the server, but that may be flagged by our ModSecurity protection. This tool is that an application will throw a 404 Not Found error when submitting some sort of change through the admin level of the site application, but block suspicious looking strings to prevent them being passed through to the server.
When this happens, our Support team needs to verify whether or not it’s a modsecurity issue.
Firstly you will need to provide to Support ALL the details necessary so that we can go through and reproduce the error on our end. This usually includes getting the URL for their site application admin login page, and the user name and password for that location. You will also want some detailed instruction on how to duplicate the error such as Navigate to the Settings Page, click on Test Post, add a period at the end of the post and click Submit.
Once you have all the information for us in order to reliably reproduce the error, please submit a request to our Tech Support. We need to have a ticket to track the issue -- so a live phone call or chat will still result in a ticket being created.