Thanks Christian – working very well now!

So sorry – I copied the wrong url.

https://heuristref.net/h6-alpha/?db=falk_playspace&website&id=12740

This little test page doesn't have much content. There's a heading and then the iframe below it. When I click on 'group by' and select a new grouping variable, I am redirected to the src of the iframe in the top window. 

Thanks for your super-quick reply, @Christian! Here you go:

https://heuristref.net/h6-alpha/?db=falk_playspace&website&id=12740&edit=2