External project dependency with eclipse problem

I have a project with my main vaadin application.
In an additional project I have a custom component.

When I add the second project to
build path
and into the
Deployment Assembly
of the first project - what
normally works in web-apps - I am getting an java.lang.NoClassDefFoundError of the classes in the second project.

How can I achieve this in eclipse with vaadin ?

looks like an known BUG

Already a few years old :sunglasses: