This is because of my PR, we changed some things in /etc/lighttpd/conf-available/50-dietpi-https.conf, but I made a mistake and now there are two options lines, which lighttpd does not like.
We can delete the “sessionticket” option, recommendation is to use only the -ServerPreference, see: https://redmine.lighttpd.net/projects/lighttpd/wiki/Docs_SSL#Perfect-Forward-Secrecy-PFS.
Note also, that there are two entries in this file, on for IPv4, around line 16, and one for IPv6 around line 33.
The problem now is, you can change this lines in /etc/lighttpd/conf-available/50-dietpi-https.conf but when you run dietpi-letsencrypt the file gets newly generated by the script, so we have to edit the script itself for now.
workaround:
sed -i '/"Options" => "-SessionTicket",/d' /boot/dietpi/dietpi-letsencrypt
sed -i '/"Options" => "-SessionTicket",/d' /etc/lighttpd/conf-available/50-dietpi-https.conf
After that you can re-run dietpi-letsencrypt.
edit: PR is out, apologies for the inconvenience