After thinking about this, and looking at the other scans you posted Marco, I felt that it was not the scanner, or you would have seen it in the Ektar 100 as well. Then thinking further back to Ektar 25, I remember how the couplers would "oil out" or crystallize in a coating on keeping. It caused much the same effect, due to the fact that when a coupler or dye crystallizes it becomes rather opaque and changes color giving light colored dots in a photo micrograph.

So, my suggestion right now is that this might be crystalline dye and coupler in the film causing light dots to appear at high magnification. If so, it is the same effect that caused the old Ektar 25 to appear to have coarser grain when kept than when fresh. Of course, in the Ektar, the effect was much worse, but still rather similar.

There are examples of this "old Ektar 25" problem posted here by other APUG members.

Maybe this is what you are seeing. Just a guess. Then again, as Ian says, it might be poor bleaching. This can be tested by re-bleaching, washing, fixing, washing and stabilzing. A rescan would then show the difference.