Problems with image assist

I've a problem using "node as block" and image_assist. When an image is included in the teaser, drupal doesn't display the image but the [img_assist] tag - in other words the "inline images" filter doesn't work.

Note that Image_assist works perfectly in all pages. I also tried to clone the website to another system (from linux/php4/mysql4 to windows/php5/mysql5) and on the new system it WORKS. So it's not something about thaT website's settings. Any suggestion please to investigate the problem?

thanks