Sync Slides to Music in the Slideshow Module

I am using LR Classic V 11.5.

I am trying to use the option "Sync Slides to Music" to create a slideshow. LR opens a window which says "Slideshow in preparation" (in German "Diashow wird vorbereitet").

The process get's stuck shortly before the end and just stays there. If I create a slideshow with the same slides and music but without the option "Sync Slides to Music" everything works fine.

Any idea what to do? I tried it on two different computers (Windows 10 and Windows 11) with the same result.

Thanks for any type of input

Peter