Issue was with UserName stored in DB was AbcXyz and we tried to logged in as abcxyz (all lower case).
Solution:
The issue was in Nop.Common.Dll on Application_AuthenticateRequest event
customer.UserName == HttpContext.Current.User.Identity.Name
Instance of HttpContext.Current.User.Identity class is created when we sign in by .Net Framework, Identity.Name is set to what ever we entered as user name like for me i can enter my user name as abcxyz or ABCXYZ OR AbCxYz etc...
To correct this problem, line has been changed to
customer.UserName.ToLower() == HttpContext.Current.User.Identity.Name.ToLower()