Thank you for the hint, @Skandha. Turns out, the Featured Image Slider is working when disabling the NextGEN Gallery plugin (version 2.2.54). During the last years NextGEN Gallery and Clean Journal Pro were working fine for me.
Although, I found several other threads in the forum regarding NextGEN Gallery and try to find a solution to have both of them working properly again. If you have any idea how to achieve this, please let me know!
Thanks again and kind regards,
Martin