Hi Tanja,
Sorry that you cannot preview, play or export Slideshow in Lightroom 6.14 as the buttons are grayed out.
Thanks F.McLion and Davidg for your help.
Tanja, I've sent you a private message, please check and respond.
Thanks,
Akash
We've tried out a few things like creating a new windows admin account with a new LR catalog or disable the GPU in device manager. But my issue still persisted.
Although I'm sure I already export videos with Lightroom 6 on this PC nothing works.
Finally I built in another graphic card in my PC (Nvidia GeForce 210 with driver from 11/2016).
Now I can activate the GPU acceleration and the slideshow also works! All buttons are active and I can export videos and view the preview!
I conclude that my GPU was too old (Adobe Lightroom GPU Troubleshooting and FAQ ).