I'll try 1+1 fix. I dont think it is dishroic fog as the emulsion is still basically opaque. Looking at the emulsion side without a light source behind the film, It still looks liken the film has not even been thru chemistry. The emulsion is still all opaque and soft as you would expect! Held up to light it is only 90% opaque. The problem is emulsion side. I can see the images, frame numbers etc underneath....the rest of the emulsion, which the fixer just cannot clear.
Did ilford upgrade delta 400 to New Delta 400 by the addition of superglue?