[Douglas_Domiteaux]

As rendering goes on, scene starts to fill with black dots, Preview screen becomes illegible with black dots.

[choo-chee]

check all tons of other posts about problems with SSS from old materials

[JorisMX]

There are some rules about dilectric and sss materials.

For example I think it was placing a SSS material in a dilectric is NOT a good idea.
I have had black dots trying this and magically solved it by redoing the materials in the end.

I'd follow choochees post and search the forum