IIS Windows Authentication Error 0xc000005e

I deployed a website (ASP.NET Framework 4.6.1) with Windows Authentication which is working fine on the server but when I try to access it from outside it asks for credentials and doesn't allow me to access it. I also noticed that if I'm logged in on the server (RDP) I can browse the website from outside. The other issue I have is the time it is taking to open the home page, around 20s.

More details: Windows Server 2008 R2 IIS 6.1 Just Windows Authentication is Enable and the Providers are (NTLM and Negotiate); Extended Protection is OFF and Enable Kernel-mode authentication is checked.

EventViewer Security An account failed to log on.

Subject: Security ID: NULL SID Account Name: - Account Domain: - Logon ID: 0x0

Logon Type: 3

Account For Which Logon Failed: Security ID: NULL SID Account Name: username Account Domain: XXXXXX

Failure Information: Failure Reason: An Error occured during Logon. Status: 0xc000005e Sub Status: 0x0

Any ideas to what the problem could be? thanks.