During setup, I wanted to use a previously generated letsencrypt certificate.
However, the dialog fields where the location of the previous certs can be entered is limited to 35 characters,
which is way to short for /etc/letsencrypt/live/<domainname>/fullchain.pem
Workaround was to manually create a link "higher up" in /etc/letsencrypt, but please increase the input field length.
Also, briefly misleading: in the dialog, you ask for "path to certificate file", but as there is no extra field for path and both certificates, this should probably mean "certificate file including full path". Maybe changing this would clarify things.