Some further investigation revealed that using other extensions objects can cause the same 'IsActive' error as above as long as the geo extension is on that page as well. So it seems to me the code is triggered by QlikView in evaluating the extensions and is not triggered by code form the extension itself, although there might be a link between the two.
Can someone please help me out?
I finally seem to have found the underlying problem. One of the SVG layers that was used in the geo extension was drawn based on a geojson file. The countries in that file each contained an id which was simply reused within the extension object itself.
These id´s happened to be just numbers and within the same range as the id´s QV uses for the objects in the webview. Hence the QvAjax.js script could not make a distinction between the two different id´s.
The fix was straight forward by just appending some characters to the id´s from the geojson file before using them as id´s for the country shapes.
The error message with 'IsActive' hasn´t popped up since. so fingers crossed!