Hedgedoc has some difficulties to communicate with the server with websockets.
The protocol falls back to web polling causing some weird behaviors during parallel editions (lost content, duplicate characters, ...)
```
websocket.js:124 WebSocket connection to 'wss://hedgedoc.softwareheritage.org/socket.io/?noteId=features&EIO=3&transport=websocket&sid=<redacted> failed: Error during WebSocket handshake: Unexpected response code: 404
```
The cause eems to be a wrong backend routing in varnish for the upgrade requests only:
```
* << BeReq >> 73173170
- Begin bereq 73173169 pipe
- BereqMethod GET
- BereqURL /socket.io/?noteId=features&EIO=3&transport=websocket&sid=<redacted>
- BereqProtocol HTTP/1.1
- BereqHeader Host: hedgedoc.softwareheritage.org
- BereqHeader Connection: Upgrade
- BereqHeader Pragma: no-cache
- BereqHeader Cache-Control: no-cache
- BereqHeader User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.75 Safari/537.36
- BereqHeader Origin: https://hedgedoc.softwareheritage.org
- BereqHeader Sec-WebSocket-Version: 13
- BereqHeader Accept-Encoding: gzip, deflate, br
- BereqHeader Accept-Language: en-US,en;q=0.9,fr;q=0.8
- BereqHeader Cookie: connect.sid=s%<redacted>; indent_type=space; space_units=4; keymap=sublime; loginstate=false
- BereqHeader Sec-WebSocket-Key: <redacted>wg==
- BereqHeader Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits
- BereqHeader X-Forwarded-For: xx.xx.xx.xx
- BereqHeader X-Varnish: 73173169
- BereqUnset Connection: Upgrade
- BereqHeader Connection: close
- VCL_call PIPE
- BereqHeader upgrade: websocket
- BereqUnset Connection: close
- BereqHeader connection: Upgrade
- BereqUnset upgrade: websocket
- BereqHeader upgrade: websocket
- BereqUnset connection: Upgrade
- BereqHeader connection: Upgrade
- VCL_return pipe
- BackendOpen 95 grafana 192.168.xx.xx 3000 192.168.xx.xx 59022 connect <---------------- wrong backend
- Timestamp Bereq: 1650561229.210203 0.000000 0.000000
- BackendClose 95 grafana close
- BereqAcct 0 0 0 0 0 0
- End
```