Well, I finally got a number of returns rejected, all with the following reason:  "This return cannot be filed at this time due to a processing error that will be corrected with our next product update on March 28, 2019.  On or after March 28, 2019, update your product, open the rejected return, review it for errors, and resubmit the return for electronic filing."

At the same time, a number of other returns were successfully accepted.  Go figure.