@Martin-Kudláček I've just experienced the exact same problem so I'm curious if the previous problem was resolved?
I've tried to troubleshoot via ssh but the only I see via /var/log/nginx/error.log is:
2020/01/01 20:02:25 [error] 17992#17992: *5 connect() failed (111: Connection refused) while connecting to upstream, client: 192.168.1.228, server: _, request: "POST /login.cgi HTTP/1.1", upstream: "fastcgi://127.0.0.1:9999", host: "192.168.1.7", referrer: "https://192.168.1.7/"
Best regards,
Henrik