Poking SD card through the vent hole helped! I did not want to remove warranty sticker.
Greeting from Gdańsk!
I was hacking and modifying my DHO924S. Often I was flashing SD with my own software (see my "signature" in my footer) and sometimes that (bad card connection) was the reason of not booting with silence on a UART (goldpins close to a SD card).
Speaking of modifying. It's possible to gain bandwidth up to ~1 GHz on this puppy (with higher noise, so it's a good idea to mod one or two channels only) and even lowering the input noise (I did it partially). But this is a very precise SMD work.
BTW. If You don't want to remove sticker, I advise to make a backup of most important files from a /rigol directory via adb and a network or usb connection.
Anyway, support from a Rigol is very very bad in my opinion. Next time I will buy Siglent at least.
Greetings from a Crackow/Kraków.