JSTree bug with ngnix - webserver

Question

A Laravel app is using the JSTree to display files.
If I get the tree under http://localhost:8000 I recive the correct tree.
We have a ngnix reverse Proxy setting to access the web site from behind a proxy.
But if I open the ngnix web site there are in some cases no data. The ajax response is correct, but JSTree doesn't render it.
Have anybody a idea?

Answer 1

First I tried out the jstree().last_error() function and it is a empty object.
Here are my header, I hope it helps:
Host: DOMAIN.de
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:45.0) Gecko/20100101 Firefox/45.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Referer: http://DOMAIN.de/explorer/show/443
Content-Length: 6
Cookie: cartalyst_sentinel=eyJpdiI6I...iJ9; laravel_session=eyJp...J9
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
The response:
Cache-Control: private, must-revalidate
Connection: keep-alive
Content-Encoding: gzip
Content-Type: text/html; charset=UTF-8
Date: Tue, 12 Apr 2016 06:37:12 GMT
Expires: -1
Host: DOMAIN.de
Pragma: no-cache
Server: nginx/1.4.6 (Ubuntu)
Set-Cookie: laravel_session=eyJpdi......3D; expires=Tue, 12-Apr-2016 08:37:35 GMT; Max-Age=7200; path=/; httponly
Transfer-Encoding: chunked
X-Powered-By: PHP/5.6.19
The PHP header:
header('Content-Type: application/json; charset=utf-8');
The problem is, with ngnix the response has another Content-Type. ngnix put "application/json" to "text/html".
Are there any options to modify this?