Hello sakinshrestha, thanks very much for coming back to me!
The problem is solved in the meantime.
The posts showed up in the slider right after I deactivated WPML and inserted the post´s IDs in the slider configuration again.
I can just guess that WPML created IDs that didn´t match. Fortunately the German
translation in Catch Box makes the use of WPML obsolete in this project anyway.
Thanks again for your offer to help and your great theme. I´ll consider your Pro-offers in the next possible project.