This is a known issue when using both portrait and landscape image files with 'Zoom to Fill Frame' checked in the Slideshow module. The only solution is to close and reopen LR,which should now allow the 'Export Video Slideshow' operation to complete. Adobe has acknowledged the issue and is working on a solution:
Interesting. I have just given this a try again, having followed all the hoohaa about exporting videos of slideshows, and I have successfully exported a 1080 HD video with both Portrait and Landscape images. I have CC LR Version 2015.10
thank you for your responses. I'm still not succesfull. when starting the export, the progress line stops, so I assume it freezes somewhere. Maybe because I have different file type incl jpeg, raw,tiff,dng.
I have solved the problem by exporting the entire series of pictures, by converting to jpeg in 72 p resolution and importing in imovie. It is an extra step, but at least I have a smooth running show.
Appreciate though to run the slideshow from LR.