Connection issue

I just installed Seafile 6.0.2 on Debian 8 (with deploy script):

# wget --no-check-certificate
# bash seafile_debian 6.0.2

I removed ufw and cleared iptables rules (I manage FW with an external appliance).
I confirm that I opened port 443/TCP from any on my FW.

While I was able to create accounts, and make some Libraries from some devices, I am having several connection issue from clients (android app, or even desktop clients).
In such cases, I run tcpdump on server and I see lots of SYN packet on server, without SYN,ACK

For example, right now I am unable to telnet to server’s ip to port 443.
Anyway, on that server, 443/TCP is listening…

root@server:~# fuser 443/tcp
443/tcp:               618   619   620
root@server:~# ps aux |grep 618
root       618  0.0  0.0 101424  1356 ?        Ss   Feb25   0:00 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
root     17753  0.0  0.0  12732  2136 pts/0    S+   12:04   0:00 grep 618

How can I solve this?
Thank you

Further details:

2017/03/02 21:42:47 [error] 12748#0: *3 upstream timed out (110: Connection timed out) while reading response header from upstream, client:, server: , request: “GET /api2/ping/ HTTP/1.1”, upstream: “fastcgi://”, host: “