Nginx proxy config - websocket connections get 403

Hi. I'm in the process of configuring nginx proxy for mattermost, and can't seem to understand what causes websocket connections to 403. Note the official nginx config published here works just fine. It's just that I'd like to use the common proxy configuration that's already defined on the server.

Conf:
mattermost site:

proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=mattermost_cache:10m max_size=3g inactive=120m use_temp_path=off;

server {
    listen 443 ssl http2;
    ssl on;
    server_name mattermost.example.com;

    location / {
        include /config/nginx/proxy.conf;
        proxy_pass http://192.168.1.100:8065;

        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";

        proxy_cache mattermost_cache;
        proxy_cache_revalidate on;
        proxy_cache_min_uses 2;
        proxy_cache_use_stale timeout;
        proxy_cache_lock on;

        access_log      /config/log/nginx/mattermost.access.log;
        error_log       /config/log/nginx/mattermost.error.log;
    }

    location /api/v3/users/websocket {
        include /config/nginx/proxy.conf;
        proxy_pass http://192.168.1.100:8065;

        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";

        access_log      /config/log/nginx/mattermost.access.log;
        error_log       /config/log/nginx/mattermost.error.log;
    }

}

/config/nginx/proxy.conf:

client_max_body_size 10m;
client_body_buffer_size 128k;

#Timeout if the real server is dead
proxy_next_upstream error timeout invalid_header http_500 http_502 http_503;

# Advanced Proxy Config
send_timeout 5m;
proxy_read_timeout 240;
proxy_send_timeout 240;
proxy_connect_timeout 240;

# Basic Proxy Config
proxy_set_header Host $host:$server_port;
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 https;
proxy_set_header X-Forwarded-Ssl on;
proxy_set_header X-Frame-Options SAMEORIGIN;
proxy_redirect  http://  $scheme://;
proxy_http_version 1.1;
proxy_set_header Connection "";
proxy_cache_bypass $cookie_session;
proxy_no_cache $cookie_session;
proxy_buffers 32 4k;

Error shown in mattermost log:

[2017/02/17 02:53:00 CET] [EROR] websocket connect err: websocket: origin not allowed
[2017/02/17 02:53:00 CET] [EROR] /api/v3/users/websocket:connect code=500 rid=rq54i7rnyprri8qgsod1kccaey uid=kmht6cdtttf67f4h8jifeiapjh ip=192.168.1.102 Failed to upgrade websocket connection [details: ]

Mattermost ver 3.6.2.