Date of success is March 7, 2021.
Ubuntu 20.10, x86_64,
Hardware: Intel HD 4000 Graphics, i5-3570, H61 Sandy Bridge chipset, essentially a Dell Optiplex 3010.
Kdenlive 20.12.2.

I tried enabling GPU acceleration (movit) in Playback settings. But kdenlive kept crashing.

Cause: (I now think) missing libraries, and a bunch of them, that kdenlive package in Ubuntu should suggest, but doesn't.

Solution was to git clone the recent version of mlt (melt), the video processing package that kdenlive overlies, and run its configure script, not with the goal of building it, but to figure out (from the configure script's error messages and warnings) what missing libraries I needed to install under Ubuntu to make kdenlive happy.

What would happen is that the mlt configure script would complain about the lack of a certain dev library. I then used synaptic to find that dev library (usually the binary was already installed), and install the dev package. Then what would happen would be that tagging the dev library for install will trigger the installation of a bunch of binary libraries.

Once I went through a cycle or two of that, so that the mlt configure script said, "yeah, I have all the libraries", THEN, I didn't even need to "make" the mlt library (already installed from stock pool on Ubuntu). It was the process of going through configuring the build of melt that wound up making me install enough libraries so that kdenlive, after that, did GPU rendered playback and all the rest without crashing or anything.

My suspicion (and I am not a good expert in any of it) is that, in the Ubuntu package system, the perfectly good existing compiled binary of kdenlive needs to have its package dependency list expanded, or something.

Anyway, went through some hours to get here, wanted to share it before I forget it all.

To reiterate, I didn't need to rebuild melt. What I did need to do was to use the act of building melt as a flashlight to illuminate a lack of missing libraries. After I installed all those, the melt that I had all along worked just fine, and kdenlive didn't crash. I can't believe I figured it out.

LOVE kdenlive.

Thank you for this detailed explanation. I link it to here: https://invent.kde.org/multimedia/kdenlive/-/issues/895 .

And these missing libraries are.......?