I tried safe-mode and disabled all customizations and plugins. Unfortunately the problem persists.
It may have something to do with another issue I noted here:
Another user said they were noticing this error in the latest version.
I just updated our instance today.
One point to note, however, is that this issue I illustrated in this particular topic (with the summary/digest emails not working) was happening before I updated our instance. That’s why I went to see if there were new updates, because I thought maybe I needed to update in order to have my summary digest emails properly working again.