Dear Friends: Good News! I have today developed 4 sheets 4x5" of Ektar 100 WITHOUT any sign of these "flakes"!!
First, as I said above, I did follow Georg's advise to use plastic bottles rather than glass (I changed for the developer and the bleach fix); I have several times filtered the chemistry; the rest close to what I did before and using the fast rotating speed on the Jobo machine.
The ONLY THING THAT CHANGED THIS TIME – with the result of NO "flakes" – is that I bought myself a new digital thermometer (Amarell Electronic, quite expensive, I think: $85.00) from the chemistry lab shop at the University, in order to have something Really Reliable about the processing temperature. It turned out that my older thermometer (belonging to the Jobo equipment) showed 1.8°C TO MUCH! Thus, my "problematic flake producing" Tetenal C-41 processing had consistently been performed at too low temperature, perhaps something like 28.2°C (82.76°F) rather than 30°C (86°F)! (Though, obviously, underdeveloped no problem producing OK scans from these "flake negs"!)
Could this too low processing temperature explain the "flakes"?? (Why no "flakes" on the first Jobo processed film using the same incorrect thermometer that came with the Jobo equipment?)