Hello,
In de manual I see this command: gpg --keyserver wwwkeys.eu.pgp.net --recv-keys C509840B96F89133
This GPG keyserver is down for a long time. I found this alternative what works and has the key: gpg --keyserver wwwkeys.stinkfoot.us.pgp.net --recv-keys C509840B96F89133
Met vriendelijke groet, Paul van der Vlis.