WHMCS stopped working after Debian update (ERROR 500 - Internal Server Error!)

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.

 

I'm not sure I understand your reply - PHP-FPM is already in place and yes it is running ISPConfig and it always had.

 

Switching to Fast-CGI resolved the issue - thank you till. However switching it back to PHP-FPM breaks the site again.

Looking into ISPConfig manual there seem to be not much of a downside of use fast-cgi over php-fpm besides inability to change the php.ini options (which is shared feature with php-fpm), however if I wanted to use PHP-FPM anyway - what would I debug/fix to be able to use PHP-FPM again?