Symptoms
Attempt to open File Manager fails with error after a few minutes of loading:
Invalid backend response. Data is not JSON
Cause
FTP server is configured to be working in passive mode. But File Manager was not and directory listing failed.
Resolution
Uncomment the line 199 in file /usr/local/pem/sysvhosts/elFinder/php/elFinderVolumeFTP.class.php on File Manager node to allow the application work in passive mode:
198 // enter passive mode if required
199 //ftp_pasv($this->connect, $this->options['mode'] == 'passive');