That was exactly the problem. Thank you!
How to create a new SD card can be found here: https://duet3d.dozuki.com/Wiki/SD_Card
I also replaced the original Sandisk ultra with the faster extreme, which resulted in a significant jump in upload speed. It was worth it for that alone.
Is there a known bug with the 3.4.0beta7+7 regarding mesh bed (G29)?
To me it looks like the result is not applied.
config.g
; Z-Probe
M558 P5 C"^io3.in" H4 F300 T45000
G31 P500 X0 Y25 Z0.99
M557 X20:320 Y26:320 P10:10
bed.g (not shown here: drive learning is performed 3 times in a row)
G1 F40000 X40 Y15 Z7
G30 P0 X40 Y40 Z-99999 ; probe near a leadscrew
G1 F40000 X40 Y310 Z7
G30 P1 X40 Y335 Z-99999 ; probe near a leadscrew
G1 F40000 X320 Y310 Z7
G30 P2 X320 Y335 Z-99999
G1 F40000 X320 Y15 Z7
G30 P3 X320 Y40 Z-99999 S4 ; probe near a leadscrew and calibrate 4 motors
G1 X170 Y145 Z7 F40000
G30 P3 X170 Y170 Z-9999