After a reboot the web interface on a QNAP TS-420 NAS was not accessible over SSL. A nmap showed the NAS was not listening on the SSL port and it was configured to force SSL connections over the standard 443 SSL-port. So trying to access it over the non-SSL port just directed me to https://<ip-address of the nas> where there was no response.
To be able to access the web interface over the non-SSL port 8080 i had to ssh into the NAS using the admin account, then I issued the following commands:
/sbin/setcfg SYSTEM "Force SSL" 0 /etc/init.d/thttpd.sh restart
After this it was possible to access the NAS web interface over the non-SSL port, i. e. http://<ip-address of the nas>:8080