[Leonatus]

Sorry for causing trouble. My system is Windows 10, and I have a main computer 64 bit and another, 32 bit.
On my main computer, I downloaded the zip-file, but at trying to integrate into Sigil it wasn't possible, because it were "no valid plugin", so I supposed it could be the standalone version. Now, I'm off from home again and have my 32 bit computer, and there the automatic download of 4.2.2 worked without problems. Of course, I tried the plugin in the same way as at home, and, here also the result is in both cases: "no problems found".
Please mind that now I have no access to my home computer, so for this time, I cannot try out anything you recommended.
By the way: the new version of Sigil (0.9.16) seems to be available only for 64 bit systems.

Of course, now I see that my misundedrstanding had been that I thought the download link were the plugin itself. sorry for that!