File explorer shows dummy instead of thumbnail preview at some layout  

Hello, all great supporters and specialists,

I encounter the problem that for some file formats the preview at the windows 10 file explorer shows a dummy icon instead of the thumbnail preview. What makes it extra complex (and I could not yet find a fix for that) is that this happens only at certain layouts of the file explore (details, list, small icons). At others ((extra) large and medium icons, content, tiles) the preview is shown correctly.

The file format seems to be picture-related: see these screenshot examples for .ico and for .jpg and .png:

.ico and small icons: https://snipboard.io/WoG5nb.jpg (preview missing)
.ico and medium icons: https://snipboard.io/Z97l1s.jpg (preview shown correctly)

.jpg/.png and details: https://snipboard.io/9PZMEA.jpg (preview missing)
.jpg/.png and medium icons: https://snipboard.io/Ky6zJm.jpg (preview shown correctly)

The default app for all those file formats is: Photos
In the folder view settings "Always show icons and never thumbnails" is obviously not checked.
In the folder view settings "Display file icon on thumbnails" is checked. (Unchecking does not fix the problem.)

I vaguely remember that there was an easy fix. But that was a long time ago and I cannot find the trick (anymore) when searching for it. Do you know the magic?

Thanks for your support in advance!

Well, I have seen it working. After an update of win 10 (quite a while ago) the thumbnails switched to dummy icons. And once I could fix it, but it turned out to be a temporary fix only....
And: (Almost) all other icons behave correctly, like office files, .pdf, .ics, audio and video files, and so on.

Just the picture (and .ico) related thumbnails cause trouble