Pass Session variable value in QueryString in ASP.Net

As you are retrieving the Int value from Query String and your query string contains non integer values then you need to pass only AdminId i.e. Session Value.

Response.Redirect("Manage Admin.aspx?AdminId=" + Session["AdminSession"]);

No need to pass the Select Query in Response.Redirect as you are using Stored procedure in SelectByAdminId() function.