The 502 error from your Usenet server is very similar to the 480 Authentication Required error. So, call your ISP and ask them what you need to do. If you can’t get your ISP on the line, try to remove your username and password from your newsreader software. That has a chance of working because this error can occur when your ISP outsources, or insources their Usenet service. It is possible that a username/password is no longer required, and sending a username to the server will cause an error.
Some ISPs contract out only part of their Usenet service. So, if you request a recent post, it will download from their server. But if it is an old post, they will forward your request to a server farm that specializes in hosting older data. That process is transparent to you – until something goes wrong. And such a glitch might throw a 502 error. In this case, you should call your ISP and complain. While you are waiting for it to be fixed, you might be able to continue downloading if you restrict yourself to newer posts. You can ask your ISP exactly what part if Usenet is offline.
If you get a lot of 502 errors, then your ISP might not be very skilled at operating a Usenet system. You might want to consider switching to a premium service like Easynews.