@MisterX What worked for me was installing libapache2-mod-fastcgi package, but as it’s not in current repository, you need to use from older version of Debian (should be in Jessie (Debian 8)). I use Ubuntu, but it works basically the same. I would also disable the proxy module. I assume you already configured ssl certificates and other things anf that problem is with fastcgi only. If not, follow the whole procedure i mention at the link at the bottom.
#add old repository
nano /etc/apt/sources.list
#replace ubuntu by debian repository
#add the following to the file, save, quit
deb http://archive.ubuntu.com/ubuntu xenial main multiverse
apt update
apt install libapache2-mod-fastcgi
#Or download the package manually and install using dpkg -i package_name
nano /etc/apache2/mods-available/fastcgi.conf
#Insert the following to the file fastcgi.conf
FastCgiExternalServer /var/www/html/urbackup/x -host 127.0.0.1:55413
a2enmod fastcgi
apachectl configtest
reboot
#If this does not help follow complete procedure as described at the following link. It’s in Czech, so either translate with Google translate or only follow the commands. https://bednarik.it/jak-nainstalovat-urbackup-server-na-ssl-https-ubuntu-18-04/
#Libor