I’ve found a few 404 errors. That’s wrong links:
/wp-content/themes/generatepress/inc/css/images/ui-bg_glass_65_ffffff_1x400.png
/wp-content/themes/generatepress/inc/css/images/ui-bg_glass_75_dadada_1x400.png
/wp-content/themes/generatepress/inc/css/images/ui-bg_glass_75_e6e6e6_1x400.png
/wp-content/themes/generatepress/inc/css/images/ui-bg_flat_75_ffffff_40x100.png
Fixed in next version. This was only happening while in the Customizer, so visitors on your live site won’t create these 404s, only you in the Customizer.