I will look at the RPi monitor, but meanwhile I made a bit of progress.
Switched to a Nokia wall charger, initially the system's video was instable, but after a bit it became stable. I'm now typing this on the roseapple.
I've now remeasured the power. This is about 50 minutes after poweron.
root@localhost:/var# cat /sys/devices/b0170000.i2c/i2c-0/0-0065/atc2603c-hwmon.0/*current*
root@localhost:/var# cat /sys/devices/b0170000.i2c/i2c-0/0-0065/atc2603c-hwmon.0/*voltage*
One other observation:
monitor settings says the res is 1280x800 @ 14 Hz.
Especially the refresh rate seems low, but I cannot select another resolution
Will reboot later to see if it is caused by the power supply being warmed up or the board.