I’m cautious when it comes to “my exact problem” as similar symptoms can have very different root causes. Login errors usually imply some problem communicating to Plex’ backend (e.g. authorizing the login) – could be a firewall or DNS issues (among other things).
Server logs might help to get this sorted.
PMS v1.25.9.5721 is the latest public release.
Plex have released a new beta earlier today (v1.26.0.xxx) – if your server is configured to look for beta updates, it’ll notify you about those, while the Asustor app store(?) is probably only publishing the public releases (see Settings > [Server Name] > General > Server update Channel).
