@qurt The affected SBC is outdated and does not support rr_ protocol emulation yet. Upgrade it using apt and then try again. See https://docs.duet3d.com/en/User_manual/Machine_configuration/DSF_RPi#installing-updates
Also note that it would be better to use the native REST protocol in SBC mode unless you need to deal with boards in standalone mode, too. See https://github.com/Duet3D/DuetSoftwareFramework/wiki/REST-API for further information.