I think i found something..
When i entered this line of code to my comments.php
<?php comment_form(); ?>
It works but it uses the default comment-form of WordPress.
My theme (a little bit older) uses it’s own comment-form build into the comments.php and looks like this.
<form action="<?php echo (defined('WP_SITEURL'))? WP_SITEURL: get_bloginfo('url'); ?>/wp-comments-post.php" method="post" id="commentform">
<fieldset>
<ul>
<li>
<label for="commentsAuthor">Naam</label>
<input id="commentsAuthor" name="author" type="text"value="<?php echo esc_attr($comment_author); ?>" required="required" />
</li>
<li>
<label for="commentsEmail">Email</label>
<input id="commentsEmail" name="email" type="text"value="<?php echo esc_attr($comment_author_email); ?>" required="required" />
</li>
<li>
</li>
<li>
<label for="commentsUrl">Website</label>
<input id="commentsUrl" name="url" type="text"value="<?php echo esc_attr($comment_author_url); ?>" />
</li>
<li>
<label for="commentsComment">Het bericht</label>
<textarea cols="60" id="commentsComment" name="comment" rows="10"></textarea>
</li>
<li>
<input id="commentsSubmit" type="submit" value="Verzenden" />
<?php comment_id_fields(); ?>
</li>
</ul>
</fieldset>
</form>
I think it will be a class or ID matching problem?