The plugin version != runtime version.
The plugin does not do much apart from loading the runtime. It gets that runtime from our servers. That runtime is only updated for the full releases.
So when you try to start a game built with 5.1.1p2 the plugin will try to check for the runtime for that version on our servers but fail to find one that matches and thus fail to start your content.
Still I am curious about what you said before. When I build to webplayer on patch version, does it fallback to full release player, and that causes errors?