In the end, the solution to my intermittent on/off problem with my toolfans on the Hymera direct drives was extremely simple.
The picture shows the solution, as the Hymera stepper driver obviously interferes with the 40mm fans. The problem was that these fans 2,4,6 and 8 not always started spinning.
I tried to exchange the fans which did not help, tested the Voltage, current , settings and so on. Everything appeared to be OK.
Strangely enough, when testing the fan off the Hymera tool, even including the duct attached, everything woked fine. Just did not work when mounted on the Hymera.
Finally, Just trying some things, I pushed a thin steel plate (NOT RVS) in between the fan and the stepper motor, and now it always works, even at 5% PWM! Problem solved!
After testing at all tools, I made 4 better fitting thin plates and mounted these at the 4 tools and no problem exists anymore, ever since!