Hi spinatwachtel and iueweb.
The gallery slider built into the theme is created differently than a default image gallery. The one caveat is that you cannot create the gallery slider using images already in your Media Library; the images must be uploaded separately into the post.
1. Set the post to the Gallery format.
2. Click the “Add Media” button, either the one above the editor or the one below the editor in the the Format:Gallery option. This will take you to the default Insert Media screen, displaying images in your Media Library.
3. Use the Insert Media screen, not the “Create Gallery” option on the left.
4. Select the Upload Files tab.
5. Upload the images you want to include in your gallery; they will be added to your Media Library.
6. When done, just below the Upload Files tab, select “Uploaded to this post” from the dropdown list. This will show the images you have uploaded that will be included in the gallery.
7. Here you can add additional images, rearrange the images, change the details or settings, or delete them.
8. That’s it. Close the Insert Media screen using the “X” in the upper righthand corner; you’ll be returned to the post editing page. Don’t click the “Insert into post” button in the lower righthand corner.
9. The images in the gallery are NOT displayed in the post editor. To see them in the slider either preview or publish the post.
10. To change the gallery select either of the “Add Media” buttons, and then “Uploaded to this post” in the dropdown list.
Let me know if you have any questions.