Hi, I did update on my Debian 8 installation: Code: # apt-get update && apt-get dist-upgrade And the WHMCS suddenly stopped working - giving only "ERROR 500 - Internal Server Error!" I switched the ISPConfig to debug log level and the logs show following: Code: [Wed Feb 28 17:20:13.425615 2018] [fastcgi:error] [pid 50830] (111)Connection refused: [client 13X.XX.XX.XX:45997] FastCGI: failed to connect to server "/var/www/clients/client1/web5/cgi-bin/php5-fcgi-1ZZ.ZZ.ZZ.ZZ-443-example.com": connect() failed [Wed Feb 28 17:20:13.425667 2018] [fastcgi:error] [pid 50830] [client 13X.XX.XX.XX:45997] FastCGI: incomplete headers (0 bytes) received from server "/var/www/clients/client1/web5/cgi-bin/php5-fcgi-1ZZ.ZZ.ZZ.ZZ-443-example.com" Notes: 13X.XX.XX.XX = my local public IP 1ZZ.ZZ.ZZ.ZZ = server IP Server is still running Debian 8.1 with PHP-FPM as it always had. Nothing apart the Debian update has been changed - it's been running for months, though I admit I didn't do the updates for a while. Any input is greatly appreciated. Till has been super helpful the first time, I hope it's an easy fix. WHMCS is nothing but trouble, so sad the ISPConfig billing module has so little payment gateway options that work out of the box I would be happy to pay much more to have those gateways working out of the box and to be able to avoid this every time.