UART0 console not working (latest Ubuntu image)

Hi,
I’m using the latest Ubuntu official image with latest kernel 4.4.143-55.
The board boots and works (I can login with SSH via Ethernet).
I want to use a serial console on UART0 at 115200 (my USB-Serial converter can not handle 1500000).
So, according to the Wiki, I have edited my /boot/uEnv.txt:

rock@rockpis:~$ cat /boot/uEnv.txt

verbosity=7
fdtfile=rockchip/rk3308-rock-pi-s.dtb
console=ttyS0,115200n8
overlays=rk3308-uart0
overlays=rk3308-usb20-host
rootuuid=e6cc3977-ae0c-4340-9b51-66197ebac83d
initrdsize=0x657e82
kernelversion=4.4.143-55-rockchip-g6b7accbc999b
initrdimg=initrd.img-4.4.143-55-rockchip-g6b7accbc999b
kernelimg=vmlinuz-4.4.143-55-rockchip-g6b7accbc999b
extraargs=swiotlb=1024

I connected my usual, trusty Usb-to-Serial converter to Header #1 (ref. https://wiki.radxa.com/RockpiS/hardware/gpio), pins:

usbserial GND <-> pin 6 = GND
usbserial RX <-> pin 8 = UART0_TX
usbserial TX <-> pin 10 = UART0_RX

The converter works happily with all my other Linux boards.

But I can see nothing with the RockPI-S.
Not during boot, not afterwise (no login prompt etc.)

Am I doing something wrong?

Thanks for reading

I tried changing the USB-Serial converter, but the problem persists, the output is a bit different: instead of a blank screen, I get garbage text:

▒f▒O▒▒▒▒▒M▒▒-],▒9fFfW▒nFF▒#gOgF▒n?FGWw▒▒▒▒F▒_KvN_o>▒vw_W▒ofkWOO~&:ow'^oo_gNFOGWGO?▒_▒N

like if baud rate was wrong (but I specified 115200 in uEnv.txt, see before)