The most likely cause is that the 3420 cannot open an FTP connection to the controller PC.
Another cause could be a hardware failure on one of the SRPMs. You could verify this by connecting the Spirent supplied SRPM cable (RS-232 on one end, mini USB on the other end). The SRPMs are the DSP boards inserted in the back of the SR3420. Connect the SRPM cable to any computer equipped with a serial port and open hyper term (settings: COM 1, 115200 Baud, 8 data bits, 1 stop bit, no parity, no hardware flow control). If the hyper terminal will not display any output, it is safe to assume that the hardware is damaged.
On the back side of the 3420, verify the 6 LEDs on each board shows an intermittent green status. A persistent red might mean a hardware failure. Contact support.
Note: It is very rare that both SRPMs on a 3420 experience a hardware failure at the same time