I actually have the same error, I was making as well a material intended to be used as volumetric fog within a particle systeme, I end with the same error as you.
The weird thing is that i have a tutorial project that i was using to learn how to make such material (it was a 4.19 project upgrated into 4.25) this one is working fine, then tried to import the content of the tutorial project into mine to check, and there the material break and give the same error.
So I tried with a fully new clean project and redo by hand the material from the tutorial project, again same error, I obviously used the launcher to check if there was no issue on the engine itself, no issue found.
This issue is a complet mystery for me.
EDIT: I found the issue on my side, it was plugin related, Gfur Pro was causing the error, will report the issue immediately to them.