I had a look on the source code of bugnet. As you can see the last thing what has to be done while installation, is to create the user administrator and add him to a role. Are you sure that the SQL Server user which you have used for bugnet has insert/modify rights on your SQL Server. Perhaps you gave him only the right to create tables.
Code:
WriteMessage(string.Format("Installing Version: {0}", GetCurrentVersion()));
WriteMessage("Installing Membership Provider...", true);
ExecuteSqlInFile(Server.MapPath("~/Install/dbscripts/InstallCommon.sql"));
ExecuteSqlInFile(Server.MapPath("~/Install/dbscripts/InstallMembership.sql"));
ExecuteSqlInFile(Server.MapPath("~/Install/dbscripts/InstallProfile.sql"));
ExecuteSqlInFile(Server.MapPath("~/Install/dbscripts/InstallRoles.sql"));
WriteMessage("Installing BugNET Database...",true);
ExecuteSqlInFile(Server.MapPath("~/Install/dbscripts/BugNET.Schema.SqlDataProvider.sql"));
WriteMessage("Installing BugNET Default Data...",true);
ExecuteSqlInFile(Server.MapPath("~/Install/dbscripts/BugNET.Data.SqlDataProvider.sql"));
WriteMessage("Creating Administrator Account...",true);
//create admin user
MembershipUser NewUser = Membership.CreateUser("admin","password","admin@yourdomain.com");
if (NewUser != null)
{
//add the admin user to the Super Users role.
Roles.AddUserToRole("Admin", Globals.SuperUserRole);
}