the image in the section by default is always 100% width, the image is 1920 pixel width, so on a 4k display of course it’s too small. Only thing you can do is to use a bigger image or to set via style attribute “background-size:100%” to the section.
For the error message: have you changed something in the php files? Or in the config.php file? Because both arrays are set in the theme files, only if you make changes this error can occur.