ok i've done a bootloader reset (holding the reset button during powerconnector insertion) and re-upgraded the firmware with the same firmware version. After that, it seems like configuration sticks after rebooting several times.
i do have new symptom now: after i select reboot from the webgui, the device does not come back. no leds are blinking or lit up, only the LAN link is up. I have to hard poweroff / poweron the device to boot it.