TUSB4020BI: The TUSB4020 and our USB nullmodem cable are detected by linux, but there is no USB communication with the Terminal

Hello, Texas team.

BACKGROUND

We are using an embedded AM3505 processor's USB0 port to control the TUSB4020. 

We use a USB nullmodem cable to allow console access from a terminal program (e.g Putty or Teraterm).

This console port is connected to port USB_DP_DN1 / USB_DM_DN1 of the TUSB4020.

This has worked satisfactorily for almost all of our PCBs.

We have two boards that exhibit a fault:

FAULT DETAILS

After power-up, we can see both the TUSB4020 and the null model cable detected by linux as expected:-

>lsusb

Bus 003 Device 003: ID 0403:6001

Bus 003 Device 002: ID 0451:8027

BUT - There is no communication with the terminal program.

QUESTION

Given that all the inputs/voltages  of a "faulty" board are the same as a working board, is there some other parameter that would stop the USB_D(P)(M)_DN1 port from working?

OR - is it likely that the TUSB device has been damaged?

Thanks,

Fraser