Oh that is interesting to hear - i might try to roll back to the previous version - i somehow didn't think of updates screwing with the admin panel.
Edit:
I've found another thread mentioning that the service lighttpd might not be running and checked:
$ service lighttpd status
● lighttpd.service - Lighttpd Daemon
Loaded: loaded (/lib/systemd/system/lighttpd.service; enabled; vendor preset: enabled)
Active: activating (auto-restart) (Result: exit-code) since Wed 2018-02-28 08:45:58 UTC; 92ms ago
Process: 4456 ExecStart=/usr/sbin/lighttpd -D -f /etc/lighttpd/lighttpd.conf (code=exited, status=255)
Process: 4446 ExecStartPre=/usr/sbin/lighttpd -tt -f /etc/lighttpd/lighttpd.conf (code=exited, status=0/SUCCESS)
Main PID: 4456 (code=exited, status=255)
Feb 28 08:45:58 raspberrypi systemd[1]: lighttpd.service: Main process exited, code=exited, status=255/n/a
Feb 28 08:45:58 raspberrypi systemd[1]: lighttpd.service: Unit entered failed state.
Feb 28 08:45:58 raspberrypi systemd[1]: lighttpd.service: Failed with result 'exit-code'.
Edit 2:
Issue was resolved by creating the httpd log directory:
sudo mkdir /var/log/lighttpd && sudo chown www-data.www-data /var/log/lighttpd
This needs to be done on each reboot and it worked fine since then!