The server is rejecting requests, why?

Question

ASP.NET

Hello, I have this situation: I
have a desktop application, they make calculations and send them to my server, Post request, not a big json (3 numbers).
The server is an asp net core application on azure.
Hosting plan is free (if it matters)
My server is forcibly closing the connection, but not on all requests.
I set a breakpoint in catch in order to see what happens, I let it try again - everything is the same, then I made a request through the postman - everything is OK, I let it try again - I closed the connection.
As a result, such a picture, the flow will either normally send a request, or it will fall and repeated attempts will not save it.
I want to say right away that I send few requests - 1 per minute.
The fidler shows a connection error.
I'll post screenshots at home.