Why my Android APP can not back up the contacts?(Use Nginx reverse proxy remote server)

I have successfully installed NC13 on my ubuntu16.04 server, Every thing looks good, And I feel NC is an amazing software.

if I strictly according to the official document (use FPM), the web page not displayed properly(But Android APP is OK) .But, if i use this config:

location / {
proxy_set_header Host $Host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;

proxy_pass; #Point the apache server, Nginx server’s ip=

The web page will be OK, At this point, just Android APP cannot backup contacts( “you don’t permission…”), If I choose to synchronize this folder, APP prompts " Forbidden".
This looks like a directory permissions problem , But believe me, just change to FPM way, as long as the FPM ways, the problem is gone (Even the web page not good). Also, I can normally use a different folder, only this folder (.Contacts-Backup) prompt a permissions issue.

When error - backup contacts APP, I checked the logs, there was no new content.(My loglevel=2). and my log function is ok.
Why is this? Please help me.

Nginx server OS: ubuntu 16.04
Apache server OS:ubuntu 16.04
App: 3.2.0

Hi everyone:

Sorry , i do not know how close this post.
I already solve my problem, it was a mistake in my Nginx config, I wrote ‘deny all’…

Thanks for everyone!