Hi @Rainerhamm, thanks for taking the time to report this!

Reproducible e.g. by not being logged in and clicking on the image in https://www.kempedia.de/index.php?title=Ellenstra%C3%9Fe_10 :

Fehler: could not load image from http://www.kempedia.de/images/thumb/3/3d/Ellenstrasse_10_2013.jpg/800px-Ellenstrasse_10_2013.jpg
and that URL is actually a 404 hence no thumbnail of that size (800px) was created to display.
https://www.kempedia.de/images/thumb/3/3d/Ellenstrasse_10_2013.jpg/250px-Ellenstrasse_10_2013.jpg exists, however.

This might be an issue with the configuration of the software that you use for creating image thumbnails, instead of MultimediaViewer itself.
Has this problem been brought up on https://www.mediawiki.org/wiki/Project:Support_desk already?

It is not only with that picture. I cannot open any picture in my wiki kempedia.de. A few weeks before (may be before the update to 1.28) it worked without any problems.
I do not use any special software for creating thumbnails. I only want to view normal uploaded pictures with the Multimediaviewer.

No, I haven't brought the problem to the Support Desk. It's my first attempt to get some help. I don't know the right procedure.

What can I do, what shall I do?
Thank you in advance.

Which software is used for creating thumbnails? (ImageMagick? Something else?)

I'd recommend posting also on the Support Desk.

I don't know what you mean with thumbnails.
These are normal uploaded jpegs. It does not depend on the source of the picture whether it is not shown. I suppose that Mediawiki or the Multimediaviewer transform the pictures inti another size if necessary. I don't know.

I will try to bring the issue to the Support Desk as well.

Thank you!!

A thumbnail is the same image but converted to a smaller size. The original size of your image is 1000px.
According to the error message of MultimediaViewer that I'm getting here (I'm writing "I'm getting here" as you have not posted yet the full complete error message that you receive), MultimediaViewer tries to display the 800px thumbnail of your 1000px image. But that 800px thumbnail has not been created. Hence that error.

What is $wgMediaViewerUseThumbnailGuessing set to?
Do you have a 404 handler set?

With $wgMediaViewerUseThumbnailGuessing = true;
it works !!!!
It was not set respectively on default.
I will check whether there is set a 404 handler. I don't know.
But first it works!

Thank you very much!

In T160514#3102358, @Rainerhamm wrote:

With $wgMediaViewerUseThumbnailGuessing = true;
it works !!!!

Huh, that's usually the other way around. Guessing is fragile; without guessing it uses the API, which should work unless there is a bug or the site is seriously misconfigured.