The most likely case for this error is that the current Software version is too old for the 1401 model in use. If you have recently upgraded the hardware you should also upgrade the Spike2 or Signal software to the latest version.
Another common cause for this error is that the 1401 Firmware is old and is not recognised by the newer software. Update the 1401 Firmware for a Micro1401 mk2 or a Power1401 mk1 or a Power1401 mk2 or contact CED to update older instruments.
Does your 1401 now work?