Had the same a couple of weeks ago. Probably rejection of one ADR without warning. We had the same due to a TAT birdstrike which resulted in rejection of ADR 2 due wrong data (TAS, W/V readout and computations)
Once on the ground the cat 3 single status message was gone because the a/c is not sensing airflow anymore so data was correct again and ADR was back in the loop. Worth while crosschecking all the pdf/nd values for all adrs