The issue is probably arsing from trying to use https://. Looking at your debug token, I see that you are running lighttpd only on port 443, so trying to access Pi-hole over http should not work at all (and should also not show the block page!). In your lighttpd log, I see the following:
2018-12-15 12:05:06: (mod_fastcgi.c.2543) FastCGI-stderr: PHP Warning: file(http://127.0.0.1/admin/scripts/pi-hole/php/queryads.php?domain=<removed>&bp): failed to open stream: Connection refused in /var/www/html/pihole/index.php on line 137
As you can see, the error is caused by the blocking page as it cannot connect the queryads.php script.
You manually tried to move your dashboard (endpoint /admin) to HTTPS, but - for whatever reason - your custom configuration does not work as https://$servername/admin is serving you the blocking page (path /var/www/html/pihole instead of /var/www/html/admin). I recommend to check your lighttpd configuration.