The ResourcesConfigurationManager manipulates resource paths manually rather than using e.g. the Path interface. When a base resource path is used with a resource filename, it ensures that the base path ends with a slash, e.g. /com/example/. But the logic for appending the resource name erroneously adds an extra slash, so that a resource name of FooBar-config, for example, results in a search for:
Interestingly this works on Windows 10 when the class path is on the file system. This must be why the bug didn't show up earlier. But when retrieved from inside a JAR file, it fails (as it would be expected to do) with the double-slash.