404 when try to access Strapi behind NGINX Proxy with sub folder variant

[Serverfrog]

Like in the title.
I configured Strapi and the NGINX according the documentation.
I also saw many posts on the Strapi forum and here on Github and most of them just switched away from sub folder and are doing sub-domain, which is not possible for me.

Strapi Admin URL is reachable but all the resources and everything else is then fetched without the sub folder prepended.