When having multiple exporters installed on Confluence 5.6.1 only one of them works. For the other exporters the user interfaces do not open or show invalid content. Which exporter is working correctly depends on the order the plugins are installed / activated during Confluence startup.
Apparently in Confluence 5.6 there cannot be multiple web resource transformer modules for the same implementation class even when the module key is different. This causes the wrong plugin scope to be injected into the web resources when serving them to the user.