Not sure about that. The FUT for the UC is the same as for most other (current) USB units. The known issue is related to the old firewire only devices where the FUT is much older and not compatible to newer macOS versions.
I think the issue here could be a missing allowance for the RME driver in macOS system settings - 'privacy & security'

Thanks for the answers. Yes it's in apple mode.
Another strange thing : i've tried the install several times since yesterday, and now it doesn't even mention anything in the privacy and security settings.
So if i understand things, i should perform the flash update on some older mac, and it should work ?

Ok this is crazy, it was actually a hardware issue, the usb socket had been quite loose for some time, so i opened the case and it was partially disconnected ! I soldered it back and it now works. I couldn't get the tin off the pads, and anyway i don't have an extra usb socket at hand, so now i have ugly wires coming off my sound card, but it works anyway !