It seems to me that the question that keeps coming forward is why the acidic nature of a fixer? The primary reason is to stop the alkaline action of the developing agent.
For that matter when one reviews the PH of Sodium Thiosulfate, it is shown to have a PH of 11. That is certainly alkaline.
When one reviews the PH of Sodium Sulfite it is shown to have a PH of 9.5. That is certainly alkaline as well.
The formula for conventional Hypo fix is 32 oz of Sodium Thiosulfate and 4 Oz of Sodium Sulfite in one gallon of water.
The formula for Kodak F6 fixer is 32 Oz of Sodium Thiosulfate, 2 Oz of Sodium Sulfite, 6 Oz of Acetic acid, 2oz of Kodalk (sodium metaborate--balanced alkali), 2 Oz of Alum to one gallon of water.
The formula for Kodak F24 fixer is 32 Oz of Sodium Thiosulfate, one Oz of Sodium Sulfite, and 3 Oz of Sodium Bisulfite to one gallon of water.
It would appear to me that the preponderance of the chemicals that make up the common fixers are certainly alkaline, with the possible exception of F6 and the acetic acid may be used to control the degree of alkalinity in that formula, rather then the formula itself being truly acidic.
The reason for the addition of Sodium Sulfite (Na2 SO3), or Sodium Bisulfite (NaHSO3) is that in the chemical reaction that occurs in the removal of undeveloped silver halides (either Silver Chloride or Silver Bromide) from the film emulsion by Sodium Thiosulfate (Na2S2O3) one of the byproducts of that chemical reaction is free Sulphur (S). The free sulphur which is released then combines with the Sodium Sulfite or the Sodium Bisulfite (as the case may be). The result of this reattachment of the free sulphur is to form Sodium Thiosulfate (Na2S2O3). The net result of this chemical reaction is lengthening the effective life of the fixer solution. This chemical reaction will continue so long as there is either the Sodium Sulfite or the Sodium Bisulfite present in solution for the free sulphur to recombine with.
While I certainly do not know the true chemical nature of all of the "brown stain"mentioned in the translated text, I certainly could believe that a portion and quite possibly a sizable portion of that stain may be the free sulphur which is a natural occuring by-product of this chemical reaction. The german text that you have translated would seem to bear out the result, consistant with the chemical reaction, of the addition of Sodium Bisulfite to the Sodium Thiosulfate in preparing a fixing bath.