@dc42 I enabled the pullup resistor for the io0.out pin, (and I found out a bad crimping contact for the X endstop... ^^').
Everthing works fine now. Thank you David.

@Marius-Breuer note further on in the Noctua white paper - "External pull-up is not necessary as the signal is pulled up to 3,3V/5V inside the fan."

However, Noctua also deprecates using a single transistor and relying on the pullup and favours a two transistor drive circuit for the PWM (also further on in the white paper). I have found with some fans a need to add an additional external pullup, and actually I've generally pulled up to 12V (though not with a Noctua fan, and I note they say don't pull to 12V) see e.g. https://forum.duet3d.com/post/306593.

ok thanks now it works!!!

@danny_v1 have you looked at the functionality provided:
https://duet3d.dozuki.com/Wiki/GCode#Section_M581_Configure_external_trigger

Any trigger number # greater then 1 causes the macro file sys/trigger#.g to be executed.

so you can setup a macro to do what you want and then use a trigger to call that macro.