Thanks for the advice Bichur.
This gets curiouser and curiouser. I removed and reinstalled the Capture docklet, in D:\Program Files\Stardock\ObjectDock\Docklets\Capture, in much the same way as all the other docklets. ObjectDock certainly finds then there!
The result is that Capture setting is restored to the way it was when delivered (downloaded). Now, if i click on Open Capture Folder... (described in the readme file as "The Capture's docklet path") I get an error message:
Windows cannot find'Docklets\Capture\\'. (followed by a suggestion that I should check the spelling, etc.)
It doesn't help to change to $[dp] instead of $[dp]\, I just get one"\" fewer in the error message. It would seem that Capture and ObjectDock Plus have different ideas as to what the docklet path is.
I have tried setting the path to null. According to the readme, that means that Capture places the captured picture in the root folder of OD (Object Desktop, or ObjectDock?). At least this time I get no error message, but still no file, not anywhere on my system.
I even tried hard-coding the path as D:\Program Files\Stardock\ObjectDock\Docklets\Capture\. Result: nothing, nada, niente, rien, ingenting, nichts, niets, zilch.