If I browse to https://developer.salesforce.com and click on Login button to login, it gives me an error page saying the followings:

There was a problem with your authentication attempt. Please try again.
If you continue to encounter problems, contact your administrator.

The URL at error page is:
https://dfc-org-production.force.com/forums/_nc_external/identity/sso/ui/
AuthorizationError?ErrorCode=No_Openid_Response&ErrorDescription=Bad+response

I tried so many times on different browsers with empty cache and even tried on different machines but still can't login. What's wrong?

I'm new to this platform and created a developer account and following the tutorial here (at the last part now):
    https://developer.salesforce.com/page/Force.com_for_ASP.NET_Developers#Creating_Your_First_Visualforce_Page

When I call the "login" method, I got an error message saying:
    INVALID_LOGIN: Invalid username or password, locked out or Customer Service Portal is not enabled

I have already enabled the Customer Portal at Setup | Customize | Customer Portal and also added myself ias the portal Administrator. My organization, postal id and login username and password are also correct. The endpoint uri is:
    https://login.salesforce.com/services/Soap/c/29.0

Here's the c# code:

var ss = new SForce.SoapClient();
var lr = ss.login(new LoginScopeHeader() { organizationId = "{15_char_id}", portalId = "{15_char_id}", }, "{userid}", "{pass}");

I'm new to this platform and created a developer account and following the tutorial here (at the last part now):
    https://developer.salesforce.com/page/Force.com_for_ASP.NET_Developers#Creating_Your_First_Visualforce_Page

When I call the "login" method, I got an error message saying:
    INVALID_LOGIN: Invalid username or password, locked out or Customer Service Portal is not enabled

I have already enabled the Customer Portal at Setup | Customize | Customer Portal and also added myself ias the portal Administrator. My organization, postal id and login username and password are also correct. The endpoint uri is:
    https://login.salesforce.com/services/Soap/c/29.0

Here's the c# code:

var ss = new SForce.SoapClient();
var lr = ss.login(new LoginScopeHeader() { organizationId = "{15_char_id}", portalId = "{15_char_id}", }, "{userid}", "{pass}");