Redirect to static resource

I am running Vaadin 7.7.6 with Spring on a tomcat 8.5.3. Sometimes, seemingly random, when i enter the application url, I get redirected to a static resource like or or even the push.js. When I get redirected to the two graphics resources I get a 404 and when I get redirected to the push.js, it is donwloaded locally. Any idea what the root cause of this seemingly strange behavior might be?
Many thanks in advance.