Thanks, I got it.
I checked it and see that the wrong context was used for the featured image: the src attribute generated the URL to the post instead of the image path. To fix it go to the post.hbs and edit the featured image code snippet by adding `{{#featuredImage}}` context:
{{#featuredImage}}
{{#if url}}
<figure
class="c-card__image {{@config.custom.cardsImagePosition}} is-img-loading">
<img
src="{{url}}"
{{#if @config.site.responsiveImages}}
{{responsiveImageAttributes 'featuredImage' srcset.cards sizes.cards}}
{{/if}}
{{ lazyload "lazy" }}
height="{{height}}"
width="{{width}}"
alt="{{alt}}">
</figure>
{{/if}}
{{/featuredImage}}