right length but wrong segments

I just exported to dvd a 'testdrive' of just under 5 minutes. The only editing I did was to cut out 15 scenes from a video (all on one track) (so razor twice, cut the scene and move the remainder to the left to connect again). When I exported it, the resulting video was the right length (well, a few seconds longer, actually), but some of the sections were not the same as the ones I had selected. It looks like kdenlive occasionally picked the wrong starting points.
Everytime I export, I get a message "The export terminated unexpectedly. Output file will probably be corrupted", when the rendering is at just under 100%.
Also, probably unrelated, the first few attempts I made very short clips (from a different source, a digitised old film), just a few seconds, and parts would turn out black.

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