I remember that @joergs5 was working on a robot kinematic K13. Is it available in the 3.5beta version?

@Karim if you move x or y before homing z, it would not work correctly
just switch line 5 and 6 to be like this,

; homez.g
; called to home the Z axis
;
; generated by RepRapFirmware Configuration Tool v3.3.13 on Fri Oct 14 2022 18:23:21 GMT-0400 (Eastern Daylight Time)
G1 X115 Y0        ; align the extruder to be above the z endstop
G91              ; relative positioning
G1 H2 Z5 F6000    ; lift Z relative to current position
G1 H1 Z-355 F1800 ; move Z down until the endstop is triggered
G1 Z5 F100        ; lift Z relative to current position
G92 Z0            ; set Z position to axis minimum (you may want to adjust this)

@Karim IF you are using a switch endstop for z, first align the extruder to be above the z endstop. I use a G1 command then the position in X and Y before switch to relative position. The position of my z endstop is (X=230,Y=0)

; homez.g

G1 X230 Y0        ; align the extruder to be above the z endstop
G91               ; relative positioning

@Karim Yes

@Karim My configuration might look different. The first config file in this post looks fine, but after switching A and B motors in the motor mapping M584 X1 Y0 Z5:6:7:8 E3. I attached a simple image for the x and y axes and where the original is on the printing bed (0,0) in the configuration
The point in green would be (350,350 ,0) after homing if you keep your current homing files.

@Karim X & Y end stops M574 define high-end M574 X2, M574 Y2 switch them to low-end M574 X1 S1 p"xstop" & M574 Y1S1 P"ystop". The homing files should move in the negative values -355. Also, did you fix the mixed in wiring A & B motors

@Karim I donot have the printer nearby. I can help if you share the homing files you are testing right now

@Karim Try the old homing file and see if it works. I changed nothing for homing x and y. But for Z I added few lines to move the toolhead above the z endstop

@dc42 I returned it, and got a new one, works with no issue

@Karim I checked my config file. B motor is X and A motor is Y. Check manual page 235