So you're getting these white spots after fixing & washing, and you've used both Ilfocolor fix (as in your very first post) as well as Kodak fixer?
Is it possible that the wash is the problem? How are you washing the film exactly? And how about my earlier suggestions about avoiding dust in the drying environment? Has that yielded anything? At least you've now narrowed it down to the final stages of the process.
Yesterday I developed two rolls of 135 film. One was processed with Ilfocolor and rinsed with distilled water before drying. The other was developed with Kodak chemistry. After finishing the bleach step, I did two rounds of water wash and checked the negatives — they were clean without any spots. However, after the fixer step, spots started to appear. I then rinsed the negatives multiple times with warm water, but it didn’t help. This morning, I found that the spots could be rubbed off by hand, which suggests they were some kind of deposit.
From what I’ve researched online, fixer can crystallize at lower temperatures. It’s been pretty cold in Sydney lately, around 5°C at night. I mixed my fixer at room temperature, about 17°C, and while stirring I noticed a lot of tiny bubbles. In contrast, the developer became completely clear once the bubbles settled after stirring. This makes me suspect that the spots might be caused by fixer crystallization leading to deposits on the negatives after fixing.