There is definitely something wrong with context mappings of the two applications
These lines are logged by Vaadin 8’s VaadinServlet
INFO [default task-2]
(VaadinServlet.java:817) - Requested resource
[/VAADIN/build/webcomponentsjs/webcomponents-loader.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.
INFO [default task-2]
(VaadinServlet.java:817) - Requested resource
[/VAADIN/build/vaadin-bundle-a0b0667b27ca27849008.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.
That indicates that due some configuration issue, Vaadin 14 initialization requests are routed to Vaadin 8 Servlet instead of Vaadin 14. Hence things go wrong.