This mod causes rapid sunset/sunrise in combination with darkness mods such as clockwork and darker nights and others. it will go from full bright to full dark in a few seconds

Unfortunately this seems to be a conflict between the two mods, as both mods are tampering with the games light levels at the same time. I will list an incompatibility.

If you want a mod that darkens nights and does not conflict with my mod, the night LUT's from AAI Industry is compatible:
https://mods.factorio.com/mod/aai-industry
Or if you just want the night LUT's a standalone version can be found here:
https://mods.factorio.com/mod/light-overhaul

do you know what mod is causing the conflict?

Both mods are incompatible, as they directly alter the brightness of the game, overriding my mods changes to brightness and for some reason this triggers rapid light changes. It's likely that most mods that alter the brightness of nights directly will conflict with my mod.
If you want to make nights darker I do highly suggest you instead use AAI Industry's darker nights setting (or the standalone Light Overhaul mod) if you just want to change the brightness. This works because AAI Industry's night does not alter the games brightness directly, instead replacing the game's darkness LUT.