Amazon RDS SQL Server Express 2012 - Fail to log-in

I recently created a new RDS of SQL Server Express 2012.

I have set the security to be publicly available and to accept all ports and incoming connections.

From some reason, I fail to connect it using my three clients: - HeidiSQL - SQL Server Management Studio - java client

In each of them I configure the end point that I see in the RDS instance. I specify the username and password that I created.

I keep getting an error that the "SQL Server does not exist"

Appreciate any help!

By the way, I had no problem connecting to the SQL Server yesterday, and I can not see any informative error logs, I also tried to restart the instance.

Update:

I see the following message in the error log, is this error sever?

"2017-05-12 20:45:34.23 Server The SQL Server Network Interface library could not register the Service Principal Name (SPN) [ MSSQLSvc/EC2AMAZ-HVNIAHV ] for the SQL Server service. Windows return code: 0xffffffff, state: 53. Failure to register a SPN might cause integrated authentication to use NTLM instead of Kerberos. This is an informational message. Further action is only required if Kerberos authentication is required by authentication policies and if the SPN has not been manually registered."

Apparently the default security group was missing a rule.

Here is the missing rule that solved the issue