When I write a test in Visual Studio, I check that it works by saving, building and then running the test it in Nunit (right click on the test then run).
Are you sure that after running one of the tests the method
NavigateTo().LogonAsCustomerAdministrator();
is taking you back to where you should be? It'd seem that the failure is due to improper navigation handler (supposing that the header element is present and found in both tests).