Hi Philippe,
Can you get the latest MaterialMart version which is released just now? You can get the latest version via
www.flexiblewebdesign.com/my-account
also, don’t forget update your Virtuemart to VM3.4.0 (which is also the latest version) -- and optionally update Joomla to J3.8.12
Click to see the full release notes of new MaterialMart v1.1.0:
www.flexiblewebdesign.com/forum-virtuemart-template-materialmart-support/3566-materialmart-updates-check-and-request#14949
FYI: The problem you are having will be fixed with the new version (it is happening due to wrongly configured dynupdate.js)
Regards