Thanks both for the responses. I have tried it with the Arduino IDE and also with avrdude manually (even tried from a Mac besides my Linux box), but neither worked. My USB UART was successful in flashing the ESP, so I think the UART & cables are working fine - also, I was successful flashing the emontx via the UART earlier in the day but something must have happened that “killed” the UART. I was looking for a way to restore the thing to a factory default state (including the boot loader), do you know if that’s possible at all?
EDIT: Forgot to mention that I have had the same error message in each case.