Websocket disconnects with 1006 error, without a reason

[Esqarrouth]

I'm logging disconnects in my web game. It seems 75% of the sessions are getting disconnected with the code 1001 (normal) and 25% are getting disconnected with the code 1006 (error). https://tools.ietf.org/html/rfc6455

Sometimes on the error reason I see this text: 

Quote

CloudFlare WebSocket Proxy restarting

But he majority of 1006 disconnects don't give any reason at all. The players just disconnect with no reason at all. This usually happens at 5-30 minutes mark while the player is actively playing the game. 

The setup I'm has these:

 - Node.js  
 - Express.js 
 - Cloudflare   
 - Digital Ocean  

- Docker
 - https://github.com/websockets/ws
 - SSL
 - Nginx timeout = 3600s
 - Ping/Pong isAlive = 30s

My question is:

1. How can I debug this problem better? 
2. What are common cases which might be causing this problem?