The firmware update from 4.6 to 5.56 made the trick and the /dev/ttyNS0 now works as expected, therefore it can be used as Modbus master as intended.
nipi@M203-sn21:~$ sudo /opt/unipi/tools/fwspi --auto Board on /dev/unipispi firmware=4.6 hardware=0.1 (B-1000) (spi 6MHz) New firmware 5.56 for device id=0 fw=4.6 REBOOTING... Board on /dev/unipispi firmware=4.6 hardware=8.1 (E-16Di_U-14Ro) (spi 6MHz) New firmware 5.56 for device id=1 fw=4.6 REBOOTING...thanks @Martin-Kudláček for your help!