After I read the news about the breach I changed my password at plex.tv. I tried to get access to the server-app on my Qnap - no chance. I downloaded the latest server version from 26th August 2022. I could install it manually on my QNAP but still no access: ERR_CONNECTION_CLOSED. No access to the Backend on my qnap, no access from apple-tv to plex on the qnap. but access o my web-account on plex.tv.
add info:
ping 192.168.2.100 (that is the qnap) works
ping 192.168.2.100:32400 (that is plex on qnap) no reply
If you follow the steps login and claim your server, you should be good.
You are at the point where you open an insecure link to the qnap using an incognito mode web browser, where you then sign into that server, where you then reclaim that server.
You will be using http://192.168.2.100:32400/web as the link you paste into your private mode browser bar.