MSSQL 2019 A connection was successfully established with the server

Question

I use Microsoft SQL Server 2019 (RTM-CU7) (KB4570012) - 15.0.4063.15 (X64) Aug 15 2020 10:48:11 Copyright (C) 2019 Microsoft Corporation Developer Edition (64-bit) on Linux (Ubuntu 18.04.5 LTS)

I develop with .net core 3.1.8 web api using Kestrel :

problem/error message: A connection was successfully established with the server, but then an error occurred during the login process. (provider: TCP Provider, error: 0 - Success)

What can be the problem?

my config file:

 [sqlagent]
 enabled = false

 [EULA]
 accepteula = Y

 [coredump]
 captureminiandfull = true
 coredumptype = full

 [network]
 forceencryption = 0
 ipaddress = xx.xx.xx.xxx

my connection string:

$"Server={ServerName};Database={DBName};User Id={UserName};Password={Password};Max Pool Size=50000;Pooling=True;"

Answer 1

I used to get this message

A connection was successfully established with the server, but then an error occurred during the login process. (provider: TCP Provider, error: 0 - Success)

on my application running on centos 8 having SQL server every now and then. When I used to log in manually to the SQL server using the same credential, then it used to start working again for some time and after a couple of hours, the same issue used to popup.
After going through the SQL error log, I found that there was quite a bit of failed login attempt for the user name. I instantly disabled/renamed that user and created another user with a very big user name to use with my application.
Now no issues.
So I believe, after multiple failed login attempts on the server, it would deny any and all requests from the same user even if the credential were correct. Possibly to keep the hackers/bots at bay. So make sure the username you are using is not common ones like sa, su, admin, etc.