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Interesting...  Let me send this off to the IMGUI developers and see if they can look into it.  Can you give me any specifics about your system?  CPU, GPU, Ram, OS, etc?

ram: 4GB os: Win10 64x cpu: i5 M520 2.40GHz if that's enough ... I have exported everything from the archive to one folder.

That is... super weird.  It seems to suggest that the dll is somehow incompatible with your system.  Your system seems like it would be more than enough to run it.  
Try this.  Sometimes windows tries to protect you from stuff.  Re-download the archive, right click on it and go to properties.  There should be an option at the bottom of the general tab called 'Unblock".  Check that box and click okay, then try extracting it and running it.

So... in the zip file that you download, there should be two files: an exe and a dll.  Those two files need to be right next to each other.  You didn't happen to move them or only extract the exe by chance, did you?