I just changed <div id=”wpcr_respond_1″> in <div class=”wpcr_respond_1″> in the file wp-customer-reviews.php and it all still works fine but no error in W3C validation anymore. This ID is only used for CSS and in some function as a placeholder.
In wp-customer-reviews-admin.php you will find:
<div id=”wpcr_respond_1″ class=”wrap”>
But here neither the class nor the id are used anywhere in code or CSS so don’t bother to change it.