When enter the nether, the shader is disabled automatically, it will not happen with other shaders like photon so it's not by the mod.
the error message attached.

could you open the chat, then click the "copy info" text, then post it here please?

This was happening to me consistently every time yesterday, but now the shader works in the nether with no errors...
The error from yesterday: composite2.fsh: composite2.fsh: 0(1001) : error C1503: undefined variable "atmosphereGround"

set the resourcepack sky setting to default. this is a little oopsie doopsie on my end lol

That seems like a plausible reason for the fact that is does not happen today anymore, at least now I know what to not turn back on.
THX