Can't add any 3rd party plugins

I have a weird issue with LMS 7.9.1 (installed on a qnap NAS) whereby I can't install third party plugins. If I select a plugin and restart as requested it just doesn't show up in the installed list. Scroll down and there it is in the uninstalled list as if I hadn't done anything. Sometimes it shows "an updated version of x plugin is available" at the top of the page after I try and install, but if I choose to update the plugin then I get the same result. It seems to happen with *any* 3rd party plugin I try to install.

I have tried disabling and enabling an already installed plugin (I tested with the flickr plugin) and that works absolutely fine.

Anyone have any ideas? It's really irritating, I just want to install spotify protocol handler!