WP Rocket plugin
The WP Rocket option Enable for iframes and videos at Settings → WP Rocket → Basic → LazyLoad must be turned Off for the reCAPTCHA element to work correctly.
The FooBox option Disable Other Lightboxes at FooBox → Settings → Advanced must be turned Off for Quform popup forms to work. The option does not affect normal forms.
W3 Total Cache
If Database Caching is enabled, go to Performance → Database Cache and at the Ignored query stems: field add a new line containing