Why is NGINX throwing a 500 error?

Question

Nginx

Good afternoon, spent those. work on the server and did not understand at what point the 500 error began to appear when I opened s1.uniq.host.
I disabled Apache2 in VestaCP a long time ago, but left NGINX (you just needed to display the inscription: "You got to 1 server." Previously, everything worked fine, but now it shows:

Internal Server Error
Oops! Something went wrong.
The server encountered an internal error or misconfiguration and was unable to complete your request.
Error Code: 500

In the logs writes this:

2020/08/02 08:49:31 [error] 9619#9619: *1887 connect() failed (111: Connection refused) while connecting to upstream, client: 31.200.224.20, server: s1.uniq.host, request: "GET / HTTP/2.0", upstream: "https://109.237.108.77:8443/", host: "s1.uniq.host"

I also noticed my old IP addresses from another hosting provider.



Domains and subdomains involved in the investigation:
1. uniq.host - works fine (Timeweb regular web hosting)
2. cp.uniq.host - works fine (Timeweb VDS)
3. s1.uniq.host - doesn't work (Dedic from Adman)

In which direction to dig? :(