If you run into the message "Generated with the default ContentController.ss template" it means it could not load any other templates. Check the classname of the homepage and that you have some sort of template for it. Should extend Page then you should have a Page.ss file.
2) HTTPResponse has been renamed to SS_HTTPResponse. That error is probably coming from a module or custom code which you have so you will need to upgrade the module of if its coming from your code you will need to update your code.
Note that 2.4.2 is out now so you might want to go straight to that rather than 2.4.1.