Is it actually advisable to use a caching plugin with your theme? I know your theme is very fast, so maybe it’s unnecessary? Because I think it’s also causing errors when I test mobile friendliness and schema on Google. Would the theme work better without that WPFC plugin?
If you prefer to use a caching plugin, then there might be some settings that need to be configured to avoid some errors – might be good to check with the plugin author on this.