Websocket reverse proxy with nginx: Error about request in server log, but works in browser

The situation

I am running Etherpad, which is proxied via nginx. Etherpad uses Websockets with Socket.io.

My nginx config is more or less this one. The location block for socket.io is this:

rewrite /CustomSubDir/socket.io/(.*) /socket.io/$1 break;
proxy_pass http://localhost:CustomPort/;
proxy_redirect   / /CustomSubDir/;
proxy_cookie_path / /CustomSubDir/;

# usual proxy header
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_set_header X-NginX-Proxy true;

# websocket
proxy_set_header Accept-Encoding "";
proxy_http_version 1.1;
# http://nginx.org/en/docs/http/websocket.html
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $connection_upgrade;
proxy_set_header Host $host;

proxy_buffers 8 32k;
proxy_buffer_size 64k;

proxy_buffering off;

What's confusing

So the good message is: Everything is working! I am therefore not asking how to get anything working.
The bas one is: Although everything works nginx always shows me error messages.

What's wrong

nginx shows me these errors in the error.log:

2016/05/24 xx:yy:zz [error] 22197#0: *12059 connect() failed (111: Connection refused) while connecting to upstream, client: SOM.IPA.DDR.EES, server: somedomain.example.com, request: "GET /CustomSubDir/socket.io/?EIO=3&transport=polling&t=1464121868147-3&sid=H2GhIY24t2a40etpAACd HTTP/2.0", upstream: "http://[::1]:CustomPort/socket.io/?EIO=3&transport=polling&t=1464121868147-3&sid=H2GhIY24t2a40etpAACd", host: "somedomain.example.com"
2016/05/24 xx:yy:zz [error] 22197#0: *12070 connect() failed (111: Connection refused) while connecting to upstream, client: SOM.IPA.DDR.EES, server: somedomain.example.com, request: "POST /CustomSubDir/socket.io/?EIO=3&transport=polling&t=1464122037998-5&sid=T-pthraR669TF2cKAACe HTTP/2.0", upstream: "http://[::1]:CustomPort/socket.io/?EIO=3&transport=polling&t=1464122037998-5&sid=T-pthraR669TF2cKAACe", host: "somedomain.example.com"

So I could trace this requests down. Here is why: 1. These are obviously Websocket requests. 2. And these are - and that's special - POST requests.

When loading an Etherpad or reconnecting to one after a lost connection only one request is made, which fulfils these requirements. I can clearly se it in the browser and can see it appearing in the nginx error log in real-time. This is the request in my browser:

200 POST https://somedomain.example.com/CustomSubDir/socket.io/?EIO=3&transport=polling&t=1464121868143-2&sid=H2GhIY24t2a40etpAACd

And it contains (e.g.) this payload:

164:42["message",{"component":"pad","type":"CLIENT_READY","padId":"CENSORED","sessionID":"null","password":null,"token":"t.qbszmj[...]","protocolVersion":2}]

The server reply is:

HTTP/2.0 200 OK
Server: nginx
Date: Tue, 24 May 2016 xx:yy:zz GMT
Content-Type: text/html
Content-Length: 2
access-control-allow-origin: *
Set-Cookie: io=H2GhIY24t[...]
X-Firefox-Spdy: h2

ok

Why I can be sure this is the culprit

I am very sure the POST request causes this. Not only because it is the only POST request with this URL when accessing the pad, I can also compare the behaviour. Because on the same server I also run Etherdraw, which works very similar, but it has one important difference: It does not seem to use POST Websocket requests.
And guess what? The error.log is empty.

My questions

So what are my questions:

1. How I can see the request to be successful in my browser (with a correct server reply!) while nginx reports me it is failing in the log?
2. How can I get rid of these error messages? AFAIK the requests do not fail...