Original title:
404 page on public defined in web.xml: Triggering a 404 will render an empty white page

Everything described behaves this way only on a public instance. On Author instances all works fine.

Description:
Defining in the web.xml not a static docroot page, but a real Magnolia page.
For example:

  <error-page>
    <error-code>404</error-code>
    <location>/demo-project/about.html</location>
  </error-page>

Calling a non existing page will return an empty white page (attention you must not be logged in!):
/demo-project/notExistingPage.html

Analysis:
It must be a problem of cache, most likely related to MAGNOLIA-5835

• It works on 5.5 author
• It works on 5.5 public when being logged in
• It works on 5.5 public when adding a parameter
  /demo-project/notExistingPage.html?something
• It is not working in 5.5 public when its is caching.

Also strange is:
Flushing the cache does not help.
No elements in cache, after request one element in cache, but the page is empty/blank.