Hello Klas,
thank you for taking a look.
I'm using that template on a J! 2.5 site. So JRegistry class shouldn't be a problem.
Even if I change simple simon index.php, lne 6+7 from
$cck = new SdRendering;
$cck->getInstance( $this->template );
to
$cck = CCK_Rendering::getInstance( $this->template );
which is the original code from seb_one index.php,
it doesn't change the behaviour. Which is strange because then the template is not using the custom class "SdRendering" at all.
Anyways, I will try minima template.