How to understand where the connection is broken?

Question

Nginx

Available : server on selectel, not vds, ubunta16 on it. Spinning nginx, node js application. The domain is connected to cloudflare.
The first problem : when a POST request with a file over 100 MB is ~ 10% downloaded, the connection is broken.
What I found out : this is definitely not node.js, definitely not nginx because there is a second server with the same configurations, a different domain, and everything works there. In addition, if you set restrictions in nginx, then:
1) logs appear on the server
2) other logs appear in the browser console.
If you send a request by ip - it passes.
Problem two : I can’t figure out how to track at what stage the connection is broken, all that is from the data is the logs in the console:

Failed to load resource: net::ERR_CONNECTION_CLOSED

UPD : before I asked the question, I disabled proxying in cloudflare (by clicking on the picture with the cloud in the DNS settings). While writing the question (about 20 minutes), the changes were pulled up somewhere and the requests began to pass.
Problem three : how to enable requests in the cloudflare settings? I understand that this is a DDOS setting and probably the problem is not solved.

Answer 1

https://support.cloudflare.com/hc/en-us/articles/2...
Depending on the plan your domain is subscribed to, Cloudflare limits data uploads to the following sizes:
100MB Free
100MB Pro
200MB Business
500MB Enterprise by default (contact Customer Support to request unlimited data uploads)

Answer 2

And why do you need to send one hundred megabyte files through cloudfdare? This is idiotic and slow