Windows server stuck checking for updates

[darcilicious]

Yes, download, stop PMS, install, and restart PMS.

Can't update, "check for updates" is greyed out?

yes - just download the current version. Exit Plex Media Server and then run the downloaded 1.15.1.791 or beta 1.15.2.793 update on top It would only be a problem if you msi files for your current versions got purged by some tidy up tools like ccCleaener - but that is a general issue with updates