Hey Nicolas,
Found the culprit. Not a good idea to insert a module (or other "more sophisticated" code) into the product description while the social plugin, particularly the Pinterest button is on.
In the "System - Hikashop Social Plugin", /plugins/system/hikashopsocial/hikashopsocial.php, line 213 says
return '<span class="hikashop_social_pinterest'.$c.'"><a href="//pinterest.com/pin/create/button/?url='.urlencode($url).'&media='.urlencode($imageUrl).'&description='.htmlspecialchars(strip_tags($element->description), ENT_COMPAT,'UTF-8').'" class="pin-it-button" count-layout="'.$count.'"><img border="0" src="//assets.pinterest.com/images/PinExt.png" title="Pin It" /></a></span>';
Means the module content as part of the product description is getting inserted there, and the module code is breaking it and messing things up badly.
Solution for now: Turning off the Pinterest button in the plugin. The module is more important in the short run, so this works here and now: see the page under Sjaak's link.
But what if someone wants both, Pinterest button and a "trickier" product description? Any possible change you can see in the social plugin that would keep everyone happy at all ends? (Other than taking the product description entirely out of the href...)