What size do I need at least to get rid of the black frame in the Diashow Module? I tried it with 1080 pixels width as this is the HD setting that is also mentioned after pressing the export button. I get a huge black frame. The frame got smaller with pics that have a width of 1600 pixels but it's still there. If I use the original 21MP images everything is fine.

The Lightroom Diashow-Module does not scale up images. So if your source material (after cropping) is smaller then the output size, you will get black framing. I suggest using the new LRTExport Plugin with LRTimelapse 3 - this will give you better results and circumvent the limitations of the slideshow module. It will even scale up smaller footage in case that's needed.

But that is what I think I did. In the Diashow Module I found your presets under "Benutzervorlagen" and I've chosen LRTimelapse 24fps. With my original files it's all good. But if I use the same images after a scale down in PS (where I did some editing of colors and contrast as well) to let's say 1600px width I get the black frame. And I am wondering about that because the HD movie to be created should only be 1080px wide. So it should be another scale down in the export of the video. I'm quite sure there's a problem in my settings. But I don't find it. I thought the export might be faster if I use smaller images. That's the only reason I'd like to use the 1600px width. This size is still big enough to work on the images in PS and, in my opinion, a lot smaller in file size to reduce workload upon exporting them as a video.

I would recommend not using the Slideshow anymore for exporting the video - instead use the LRTExport module of LRTimelapse 3.
I would not recommend either to make manual changes in Photoshop single frames, instead try to make those changes in Lightroom and use the LRTimelapse workflow. The way you mix up pixel-editing in Photoshop and metadata adjustments in Lightroom (i.e. crop) might mess up things.

Ha! I watched your tutorial again. And starting at 17:20 you introduce the new export feature. I missed that! Thank you very much for drawing attention to that. Now it should work. And even better than in previous versions. You are the man! Note to myself: Watch the ENTIRE turorial first...

Btw: I created droplets with PS to do the same changes (contrast and noise reduction with Neatimage PS-Plugin) to every single frame. Sometimes the Auto-Contrast in PS just is better than my tries in LR. I did this as a test to see wether the video flickers after it or if PS is really able to do exactly the same things to every frame even if the light conditions change.

Once again: Thank you for your help!

Stephan