I am also having this problem with 4.1. I inherited from CustomerController and only overridden the Edit action. The problem is that even when the List action is called, I get the same ambiguous exception as the original poster did.
Like other plugins, my plugin adds some custom settings when installed, and removes them when uninstalled. If I release a new version of the plugin with additional settings or fewer settings required, how do I add or remove these settings without deleting the current customer's configuration on the other settings from my plugin?