This is a 2.3 regression. Downloadable resources like the link below fail.

$req.contextPath/download/resources/com.atlassian.confluence.themes.clickr/images/dotted.gif

Using a module key of any module in the plugin works, but this shouldn't be necessary.

Solution should include an acceptance test that uploads a plugin with various resources and ensure that the relevant resources can be downloaded (i.e. don't return 500 errors).