John found out that the problem was caused by another extension wich had some effect on the Joomla SEF. Since this plugin (part of a component) also does some positive things , I decided to create a menu-item for each listed article in a hidden menu. Doing so both the JAMP-plugin and the "problem"-plugin can be active plus it gives me cleaner url's as a bonus.
I wan't to thank John for his help and will give a positive review in the JED!

Kind Regards,
Jos Frenken