I cleared the cache once again and… no more problems. Does this have anything to do with new pending comments from the Fediverse? Or something else?
I may switch to a different caching plugin. I remember reading somewhere that Matthias Pfefferle had specific recommendations about which caching plugins work well with the ActivityPub plugin. To be continued…
@ele Yay, it works! I know that dance - like having to switch plugins for social media feeds on customer websites every year or two because something breaks down and/or somebody goes out of business...