Now update the 6655 to the latest and use it to initiate the login failure with the Network trace running, then compare it to another 6655 that doesn't get locked out side by side in Wireshark. (This isn't a possibility in the 75XX)
That is done like this
Of course the update itself to the device may fix it. In which case the same may be said of the 75XX, which can be obtained here.