PHPZip\Zip\Exception\BufferNotEmpty thrown

Unable to send 'febbraio-23-speranza.epub'. Output buffer contains the following text (typically warning or errors):
Warning: imagecreatefromstring(): Data is not in a recognized format in /web/htdocs/www.santalfonsoedintorni.it/home/wp-content/plugins/read-offline/vendor/grandt/phpepub/src/PHPePub/Helpers/ImageHelper.php on line 251