Maybe it's the amount of memory? With large populations it may get near or over the max memory available. So that would depend on your machine.
Btw. if you setup a file like this, there are some things I noticed that are worth considering;
1. It's more efficient to locate your pops in front of the camera for a still, and not at the same location. More than half the instances would be behind camera, taking memory.
2. If you use a fractal's color to drive displacement, it's better to reduce the color roughness from 5 to maybe 1, or you get a lot of small spikes. They take more render time, and will probably not even be seen in the kind of image you're making.
3. Also, with this kind of image, detail of 5 would be enough, and you can probably even get away with 4.
4. AA of 6 would be enough too, and perhaps even lower. Also depends on the specs of your machine, if it's not too fast, efficiency in this kind of settings is very important.
5. Easy clouds are relatively complex, and thus slow. For such high, small clouds, you could also get away with a simple v2 cloud layer and some clouds of medium size, say 400/2000/10.