Okay, I figured out why I got an “Illegal instruction” failure. It was because the installed version of “libaugeas0” did not support ARMv6 (I am running a Raspberry Pi Model B+). I could solve this by following the instructions over here: https://github.com/certbot/certbot/issues/2673.
Everything working fine now and I have installed a Letsencrypt certificate on my Pi :-).