When the unit starts up, it does readback tests of some peripheral chips, in this case the A/D and D/A converters. The message you describe means that it failed this test and did not read back what it expected.
This message could have many causes, from a bad chip to faulty reading or writing. If you have no other symptoms, you can ignore it for now, but there is always the risk that it will become a problem later.
Note that the DSP4000 was designed in the early 90's, and many of its parts date from that era.
