@martin7404
Yes I took me some time to understand the matrix concept the M669 description refers to.
I use
M669 K0 Y1:1:0:1
K0 is for Cartesian
The rest is about which axis interacts with which.
On the Muldex, Y movement causes X and U to move with a ratio of 1 and the other axes movements are considered independent.
This should speak better than words:
I wonder why no-one replied about the fact that you get a different behaviour when starting from power off or when rebooting after loading config.g.
Same thing about the upload and print function which causes crashes.
I would have thought these hints would point towards a limited number of possible causes.
I imagine some config detail could sometimes be overwritten depending on the sequence the config files or macro are loaded...
I thought about something else:
We have a difference in our M669 (kinematics type config).
You have
M669 K0 Y1:-1:0:1
As I didn't really understand why there was a minus sign regarding Y to Y reaction, I didn't go for it and used this instead:
M669 K0 Y1:1:0:1
I think the only other implication is that I need to have my Y motor directions reversed compared to yours in the M569 statements.
Speaking about it here in case the negative sign in M669 would be causing your issues somehow.
@3DPrintingWorld said in Error: bad Command XXX W/RRF 3.2 and leveling issue:
I was sitting behind the printer when I typed it.....
Lol that makes much sense!
I also get the error when selecting "upload and start".
I assume it is then running the the start Gcode from the slicer. Does it just call G28, so homeall.g, or does it call single axes homing macros for instance with G28 Y, G28 XU... Asking because only homeall.g is posted here for review.
it only needs to be defined once in config.g if it's creating an axis.
Thanks @Phaedrux for correcting my wrong assumptions!
@3DPrintingWorld Time to try my sensorless Y homing sequence 
In both config.g and homeall.g the comments about Y drives say that drive 1 is right and 2 is left.
However the separated Y homing expects "zstop" when driving the left motor, which I would call X side, not U side:
M584 Y2 ; Y-axis Left side
M574 Y2 S1 P"zstop" ; Y-axis left side endstop Active
Also, from this page:
"Any M584 commands used to remap drivers must come before M350 and M906 commands. M584 commands that create new axes must also come before the M92, M201, M203, M208 and M566 that set parameters for those axes."
As you remap Y drives with M584 in homeall.g, you should maybe redefine the mentionned settings for the Y axis after that.
Not sure of anything here, just looking around the Y homing process because you can repeat faults there.
@3DPrintingWorld said in Error: bad Command XXX W/RRF 3.2 and leveling issue:
Note that the U axis switch is connected to the Z axis endstop port.
From your config you posted earlier it seems the U axis endstop is "duex.e2stop" and "zstop" is one of the Y axis endstops.
Did you mean the U side endstop of the Y axis?