I exported the video (a rather huge video), 1080p with 29,997 fps. The video looked alright in the project monitor and timeline of KDEnlive (except the transitions stop both (transitioning) videos for whatever reason). On export there are some transitions which do not work, because the video-clips got moved around by some seconds without their grouped audio clips. Therefore I have even black video in the rendered clip, also titles got moved around. 

Reproducible: Didn't try

Steps to Reproduce:
1. Create a very long video
2. Save it -> sometimes even then the video got screwed with
3. Render it

Actual Results:  
Video clips got apparantly moved around, resulting in non-sense transitions and in some places black video

Expected Results:  
No moving of clips, no black videos, working transitions, everything keeps its place

Probably rounding-errors when saving? Do you save videos according to their frames or seconds?