If you are using 2D Shadow Maps, try disabling that. If that works, you can either not use 2D shadow maps (they help a lot on render time in some cases, in other cases not at all). Or increase the shadow map resolution and blur radius.
If you're using "Use voxels for shadows", try disabling that. If the voxel shadows are in use, you can increase cloud quality to improve the flickering/popping effect, but since that will increase render time anyway, it may be better just to disable voxel shadows. If you need to increase quality overall anyway (to reduce noise in your clouds or add detail), then try keeping voxel shadows enabled and increasing quality...
- Oshyan