When you launch an app (Flask, Dash, or Shiny) and the app directs to a page with a 404 Not Found error that says nginx underneath like shown here:
it's possible that there is a security setting in Domino that is turned on. In this circumstance, it's possible that some elements of the app are being blocked from rendering. A Domino admin can confirm this by checking the Feature Flag:
This Feature Flag controls iFrame security for the web application by adding the sandbox attribute to iFrame. To enable the application elements to render, the Flag can be set to False.
If you have any remaining questions on this issue, please contact Domino Support at email@example.com.