Thanks for your reply! Sure, the easiest way would be to just toss out the old dev kit and start with a fresh kit. The kit which causes the problems is only half a year old, I used the previous kit over a whole year without problems. I did store the previous kit in my fridge, not in the basement, so that may be the difference, I just don't like the idea of smelly poisonous chemicals in my fridge. The thing that surprised me most was that it seems to be the fixer which changes, I would have expected the FD or CD to be the first ones to go bad. Since colors and brightness are ok, I assume that FD, CD and BX1 are still in good working condition.
I will try refixing the leader of that film roll in b&w fixer, I am curious whether it makes a (positive and visible) difference or not. To my untrained eye this recipe doesn't look all that different from b&w fixer. I will report on my progress.