Bad News, I got "flakes" again! Developed 4 sheets 4x5" Ektar 100 yesterday, carefully processed them the same way as some days before with the happy result of no rainbow coloured "flakes" – this time at least some 10 flakes on each sheet, though quite small. Don't know what to do with this shit!
Perhaps there is something with the Ektar 100 emulsion, particularly in combination with Tetenal C-41 and 30°C process temperature, as some of you have suggested. Next time I will try the ordinary 38°C (100°F) style, and I will also try some other emulsion. (At least one good thing with these "flakes" is that the picture isn't totally ruined, in most cases it's not possible to detect them after scanning even when strongly enlarged. But quite puzzling, and disturbing!, that these "flakes" come and go.)
You will probably hear more from me about this – sorry!