hrst: Yes, I agree, I would expect that low processing temperature should just cause incorrect development, rather than these "flakes". Now, everything went much better when I used freshed mixed chemicals; though the first two sheets with the new chemicals still had some small "flakes", they now seem to have gone (let's hope!). Carefully inspecting these 4 dry sheets from today, I can't see a single trace of these ugly "flakes".
I suppose the best explanation (!?) is that something went wrong with the old chemicals (depending on age, bottles, or dirty equipment ?) and that "flakes" were around for some time, hard to get rid of, needing filtration and a lot of washing of bottles and other equipment. Yes, I used tap water, but next time I will use distilled/technical water – just to be sure.
My 4x5" sheets of Ektar 100 – with and without "flakes" – are from the same box (and some "flake negs" from another box but the same emulsion, newly bought and valid till 02/2012).
No doubt, nice to have some well developed very clean 4x5" colour negatives - my very first! Hope it will be more of them, but no more "flakes"!