Sorry — you’re absolutely right! I can’t explain why experiments seemed to pin the blame on Jetpack, but long after I deleted it the problem recurred — with W3 Total Cache as the real culprit! And this conflict has been documented before. With W3 Total Cache gone and Jetpack restored, the problem hasn’t resurfaced.
Thank you for your help, Janis, and sorry for the false allegation, Jetpack!
