Me again, with more issues.
I started having issues that are very hard to explain yet very simple. When I try to render something, it just shows this window. It renders in viewport though.
silhouetterainbow.blend (1.1 MB)
I can’t even render a still of this, without Blender coming to a complete halt.
But I will note that having an output MPEG size of 5000x5000, for 1000 frames, at 240 FPS - with everything inside a volumetric cube - seems … unique.
Also, Fwiw: you have no viewer node in the compositor.
I haven’t checked the file, but based on @thorn comment the first thing I’d suggest is to render to an image sequence instead of a video, and, if possible avoid such a huge output size, unless you’re going to project that video on a massive billboard I doubt you’ll need something that big. And even if the video is meant to be projected on a billboard you can get away with a lower resolution anyway.
very unique 
I have tried rendering a 10x10 image just to see if that was the issue, and it couldn’t even render that.
5000x5000 is not a valid FFMPEG output size. It has to be recursively divisible by two in both dimensions
There again, 10x10 is not a valid output size.
Oh. One moment while I go laugh at myself and try it again.
No worries, it’s an obscure and esoteric limitation 
