If you use your API URL:
http://vaadin.com/api/8.1.5/
as link for javadoc generation, it won’t work because: javadoc: warning - Error fetching URL: http://vaadin.com/api/8.1.5/package-list
The package-list returns as download in the browser, but it should return the content of the file. I guess the problem is the content-disposition?
With other API docs, like: http://docs.oracle.com/javase/8/docs/api/package-list the response contains the package list - without content-disposition…
Currently, it’s not possible to use the official API reference with javadoc.