Controller Action 404
When your plugin has a camel cased name you need to make sure you camel case your references to it, such as when you try and visit an action url.

I assume this is due to when craft tries to resolve the url it sees pluginname and pluginName as two different things, much like php variables. 

I would also assume this is due to the way Craft autoloads classes.

So using webMentionReciever would resolve since it would be a "exact" match to your plugin name.

This isn't limited to just plugin names, the same goes for any service, controller etc references too.