New installation does not serve javascript to the browser

Cloned the repo., built it apparently without any errors, then ran:

   $ mycollab-app-community/target/staging/bin/startup.sh

it prints a lot of `INFO` logging but when I browse (on the same machine) to http://localhost:8080 it logs:

   16:39:03,214 INFO com.vaadin.server.VaadinServlet:825 - Requested resource [/VAADIN/widgetsets/com.mycollab.widgetset.MyCollabWidgetSet/2F3BD702BB87447504278ED3BED13696.cache.js] not found from filesystem or through class loader. Add widgetset and/or theme JAR to your classpath or add files to WebContent/VAADIN folder.

and the browser page shows a 'spinning petals' waiting indicator but nothing else.

Any ideas?