You assume there will be two elements in the string array returned by Split function, and you take the second element (index 1).
But, if the array has zero or one element only, you get an ArgumentOutOfRangeException, telling you that you tried to access the second element of an array that holds less than two elements.
Solution: validate that the array you are trying to access is actually of the length you think it is.
Something like:
string[] parts = HttpContext.Current.User.Identity.Name.ToString().Split('\\');
if (parts.Length == 2) {
HttpUsrName = parts[1];
}
else {
HttpUsrName = "Problem in username parsing from http context";
}