xrandr: Configure crtc 0 failed (How to setup 4k with PI400)

[pi400user]

$ cvt 3840 2160
# 3840x2160 59.98 Hz (CVT 8.29M9) hsync: 134.18 kHz; pclk: 712.75 MHz
Modeline "3840x2160_60.00"  712.75  3840 4160 4576 5312  2160 2163 2168 2237 -hsync +vsync

 

$ xrandr --output HDMI-1 --mode 3840x2160_60.00
xrandr: Configure crtc 0 failed

 

$ xrandr --verbose
Screen 0: minimum 320 x 200, current 3840 x 2160, maximum 7680 x 7680
HDMI-1 connected primary 3840x2160+0+0 (0x41) normal (normal left inverted right x axis y axis) 708mm x 399mm
...
CRTC:       0
CRTCs:      0

 

I would like to setup 4k with a PI400. I found https://docs.armbian.com/User-Guide_Fine-Tuning/ and followed the instructions Did I something wrong, or is this simply a bug?