Edit: The regex’s isn’t correctly formated. I tweeted Yoast about this, in meanwhile you can fix this by commenting the lines 1434 to 1437 in plugins/wordpress-seo/admin/class-metabox.php.
/* foreach ( $matches[0] as $img ) {
if ( preg_match( '|alt=("|\')(.*?)\1|', $img, $alt ) )
$imgs['alts'][] = $this->strtolower_utf8( $alt[2] );
} */