I installed the latest version of ZP and this plugin, but all I get is an empty page when I click on the button. What could be the reason for this?

As always if you get an empty page there likely an error occured.

Yes! That must be it! Thank you very much!
And... any idea what could be causing this error?

What error?...;-)

It must be the heat... ;-)
Anyway, in 2.9 I could use this plugin, but now for some reason not anymore.

Since this plugin is now unsupported I don't know. Bascially the image tab does much what this formerly did. Besides you might want to download the recent version. I think sbillard had updated it before moving it to unsupported.