This belongs into the top 10 FAQs.
You can not simply place a GUI web app under a path, even if you use strip-prefix middleware.
The first page might load, but it usually contains links, script and images with absolute path names, so they can’t be loaded from the browser.
Check your browser developer tools network tab, you will see many load errors.
It only works when some kind of "base path" can be set in the GUI web application.
Best practice is to use a sub-domain instead, so the web application can run on root (/) path.