Okay, I see the sense in then fuzzy does, i just misunderstood the description.
Concerning the error, I have no idea what i did to get in this state. I was pretty sure that it is not caused by the plugin in itself but it's the only one to cause this error.
I just start calibre click the find duplicate button and i get the error I already posted:
I checked the debug mode, there are no errors or other infos on the console when this message appears.