Using a nRF52840 Dongle (PCA10059) in bootloader mode under Ubuntu 18.04, nRF Connect v3.3.0 sometimes stops with this message "Using USB SDFU protocol to communicate with target", but no clue about why it does not continue discovering the device's versions.
Maybe some more logging is needed after this line:
https://github.com/NordicSemiconductor/pc-nrfconnect-programmer/blob/v1.3.1/lib/actions/usbsdfuTargetActions.js#L125