I hope this will be legible without subscripts...
Sulfur chemistry is an interesting field, which we use far more than we realise.
Viradon is a solution of sodium polysulfide, Na2S(n) (the 2 and the (n) should be subscript - n simply means "any number from 1 and up")
Sodium sulfite is Na2SO3 - again numbers in subscript. Just assume that from now on?
Sodium sulfate is Na2SO4.
Sodium thiosulfate is Na2S2O3.
Sodium sulfide is Na2S.
Polysulfides tend to decompose to sulfur and sulfide, as in Na2S(n) -> Na2S + S(n-1).
The sulfur is the milky layer on the print, as well as the milkiness of the toner.
Sodium sulfite reacts with sulfur to form thiosulfate, Na2SO3 + S -> Na2S2O3.
It also reacts with oxygen to form sulfate, 2 Na2SO3 + O2 -> 2 Na2SO4.
Thiosulfate can react with oxygen too, to form sulfate and sulfur. That's why alum/hypo toners work: 2 Na2S3O3 + O2 -> 2 Na2SO4 + 2 S.
So what happened was that the sulfite carried over to the toner "absorbed" the free sulfur in the solution, turning into thiosulfate in the process. After too much sulfite has been added, there is no sulfide left to react with the silver, and the toner stops working!