M4 doesn't work

10 bit IN TRUE halui.spindle.0.override.count-enable
10 s32 IN 0 halui.spindle.0.override.counts <== SpindleOverride

Looks like you have your override active and turned all the way down which is like issuing
M4 S0

This will not activate spindle.0.on but it will activate halui.spindle.0.runs-backward.

Maybe check the value of this signal when you issue M4 S...:

net SpindleOverride <= halui.spindle.0.override.counts <= arduino.lpoti.1.out

[edit]
Actually, if you are using a potentiometer then you may want to use halui.spindle.0.override.direct-value rather than .counts which would probably be used with an encoder.

So basically the issue seems to be that the M4 S.. command turns into M4 S0 which gives me the same behavior on an axis simulation config.

I presume that you have tried M4 S100 in a simulation config and checked the value of spindle.0.speed-out.

If that works then I'm afraid the only strategy I can think of is to start with a new config and add things step by step to find out what actually breaks the M4 functionality.

Unfortunately i too have the feeling, it is something with my config and or my machine, not a LinuxCNC Bug.

Fortunately for everybody else, that is likely the case. Although I'd sure like to know what is going on there.