Bertil,

you observations and conclusions sound pretty convincing, something is rotten in the state of stabilizer Since Photo Engineer posted the recipe for stab and it contains only easy to get chemicals, you could mix some home brew stabilizer and check whether this home brew produces the same flakes or not. If you still get those flakes, one of your previous process steps seems to mistreat the film so that is disintegrates in the stabilizer, if you get no flakes (more likely from what I read), I'd say it's time to confront Tetenal with this. I have successfully communicated with Tetenal several times, it can take a while to get their first response, but after that they are usually quick and helpful.

Good luck with your experiments!