The server is in the Crimea, and it seems to have been blocked due to political problems. With the help of VPN, I began to receive updatesHi Sunrise,
You are getting a 403 Forbidden error - definition of a 403 Forbidden error is:
"A web server may return an HTTP 403 Forbidden status in response to a request from a client for a web page or it may indicate that the server can be reached and process the request but refuses to take any further action. HTTP status code403 responses are the result of the web server being configured to deny access to the requested resource by the client."
I think you need to double check your http/https ports for the system, this will normally be 5000/5001 as standard or 80/443 unless you have configured custom ports.
That being said you may not be able to access the 3CX update servers there are several good ways to troubleshoot this issue in the link here, please take a look (this link had been solved):
https://www.3cx.com/community/threads/3cx-updates.47477/
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.