Magnascha

I think that indicates that you have Placed or Pasted an image, and then you have resized it without maintaining its original aspect ratio.
If you've done that, then when you look at a page, and select the image, you'll see something like this in the Context Toolbar, showing unequal DPI settings:
To fix it, double-click on the message in PreFlight, and it will take you to the image. Then click the down-arrow in the Context Toolbar, and click Original Size. Then, if that's not the size you want, readjust the size, taking care to preserve the aspect ratio. That will mean either not pressing Shift while you do it, or pressing Shift while you do it, depending on your Preference settings:

You can always Ignore. Then check the export to see if it is okay. The images will be stretched in one axis and that is why the warning is coming up, you may have done this intentionally so ignoring the warning is one solution. A better solution would be to render the images in place in a copy of the Publisher document first then export that.

Hi @Magnascha,
Sorry to hear you're having trouble!
The issues you've reported can happen when your GPU drivers are out of date, in order for me to advise which to download, can you please open Affinity Photo then navigate to Edit > Preferences > Performance and provide a screenshot of your settings here?
When working in a document and these issues occur, can you please confirm for me:
What is the physical size of the canvas? Does this occur in one specific file, or any document? Does this happen when using a specific tool / workflow, or seemingly randomly? Do you have many other open applications when editing in Affinity Photo? Many thanks in advance