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.