Notepad++ is latest version v7.3.1. Plugin Manager shows v1.3.5 but says v1.4.3 is available and always wants to update. But when the attempt is made, it fails "Installation of Plugin Manager failed.".
Another SU article Notepad++ Installation Error: Installation of plugin failed has an answer indicating that a reinstall of Plugin Manager corrected this. However, when I go to the "Installed" tab, Plugin Manager isn't even listed. I suspect this may be a symptom of the issue.
Ok, very weird. I've found that after a clean install, upon first running Notepad++, it shows no plugins under [Available], but it shows Plugin Manager under [Installed]. If I then run it as administrator, it will show a list of plugins under [Available] and [Plugin Manager] no longer shows under Installed.