Can't connect to default container

I’m trying to run Shinyproxy on an Ubuntu 18.04 server with nginx. I can run docker images without any problems but whenever I try to run shinyproxy (which correctly shows the login page) I always encounter the following errors:

http://localhost:6900/containers/create: 413, body: Maximum upload size exceeded
... 106 more
Caused by: HTTP 413 Request Entity Too Large

I’ve already changed my nginx configuration in order to allow files as big as 100M, but nothing changed.

Then I tried the same docker image locally on Windows 10, without any proxy, and still I receive the same error message. What am I missing?