Please provide a link to the exact product of USB RS485 converter you're using. The schematic of the cable and how the DE pin is signaled might be essential. It might be necessary to bring the RS485 bus to a defined state if no driver is enabled. I use one set of resistors on the bus to pull the two data wires up/down.
Edit: The ModBus message CRC is OK if the leading and trailing zero bytes are removed.