EPi CMS: Error when login to CMS

When I navigate to CMS login page. I enter username and password then I get this error:

[ArgumentNullException: Value cannot be null.
Parameter name: username]
   System.Web.Util.SecUtility.CheckParameter(String& param, Boolean checkForNull, Boolean checkIfEmpty, Boolean checkForCommas, Int32 maxSize, String paramName) +11773375
   System.Web.Profile.ProfileManager.DeleteProfile(String username) +62
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +142
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +92

Anyone have any ideas?

In advance, thanks for your support.

Kind regards,

Tan