THX for the input, especially jumping the connector.
With
M409 K"endstops.sensors"
I observed the input working.
I changed my config.g from
M574 X1 S1 P"20.io1.in" # old
M574 X1 S1 P"^20.io1.in" # new
to activate the pull up resistor.
Like this its back working.
Probably I have messed the line up, can't explain when or why though ...
Thx