In CCP, navigate to the File Manager of a website. WebFileManager does not work with the error message: "Invalid backend response. Data is not JSON."
With enabled php logging on the WebFileManager server the following can be seen in the log:
[01-Jan-2017 10:00:00 UTC] PHP Fatal error: Call to a member function getTimestamp() on a non-object in C:\Program Files (x86)\SWsoft\PEM\sysvhosts\elFinder\php\elFinderVolumeFTPIIS.class.php on line 59
The WebFileManager expects the date to be in the 'm-d-y' format in FTP output, with lowercase 'y' meaning a two-digit year, but the IIS webserver that hosts the website is sending the date in another format.
Make sure that the parameter IIS > Sites > PEM FTP Server for Shared Hosting > FTP Directory Browsing > Directory Listing Options > Four-digit years on the IIS node that runs the problem website is switched off.