DirBlur render problem

I have a render problem.

I animated a SVG with directional blur on top of a background video.

Somehow the preview looks good but not the render.

Progress status after the render says “Finished”.

I got an error in the background video but I find it unrelated.

How can I make the directional blur in the render as in the preview?

Natron Preview:

Video Render

Unrelated error:

It could be a bug in DirBlur, which can’t be debugged unless we have a project excerpt (one frame would suffice).
My guess, however, is that you have wrong alpha values (larger than 1) where the white dust appears, and when writing to a video file Natron will unpremult alpha (i.e. multiply RGB by Alpha), causing that effect.
Concerning the “unrelated error”, the video reader and writers in Natron are not bug-free, so YMMV when you use videos as inputs/outputs, and the best use is to convert your video to individual frames, do your compositing job, render to frames, and then encode the resulting video. This is the best way to get a professional result. All you need is more disk space.