OK, it is fixed. If you use Cloudways (Managed Cloud Hosting Platform) to host your WordPress, you need to contact support to change your NGINX config to allow this plugin to do its work.
Cloudways uses NGINX to serve static content. This is the list of content from the NGINX config:
location ~* ^.+\.(html|htm|jpg|jpeg|gif|png|ico|css|zip|tgz|gz|bz2|pdf|odt|txt|tar|bmp|rtf|js|swf|avi|mp4|mp3|ogg|flv|svg)
Therefore, Cloudways support had to remove the image extensions that this plugin will convert to WebP. Now, the images are being rewritten with Apache and served as WebP, while all the other static content is still being served by NGINX.
Thanks
