Although this thread is already some weeks old - I seem to be faced with the same problem:I'm mixing pictures and small videos in my holiday slide shows, which gives very nice effects.
Both, openELEC and LibreELEC until now always run these slide shows without any issues, even with different video and image formats from different cameras - awesome!
However since the update to LibreELEC 8/Kodi 17 videos don't show up any more. Instead, a small camera-icon is displayed and the screen stays black for the run time of the video clip. The slide show continues with the pictures.
Started alone (from the "Pictures"-category) all videos run perfectly.
I have a same problem. I'm using Kodi (PC, Openelec, Libreelec) for years. Until this LibreELEC 8/Kodi 17 version (on RPi2) the slideshow was great. Now the pictures run well, but when the video file have to start I can't see anything, just the audio chanel works.
Started alone all videos run perfectly.