I just had an interesting issue where the ActivityPub plugin started reporting that my author page on both this site and Worlds In Miniature was no longer serving valid JSON, and so was inaccessible:
Sure enough, checking the URL with cURL, or an HTTP client like Thunder gave a 200 response but no response body. It had been several days since I had last made any changes behind the scenes, and everything had been working in the interim, so I was a bit stumped.
Anyway, after a bit of digging around, I came upon this thread and specifically this post which pointed me to the problem: Jetpack Boost. It wasn’t the most recent plugin I’d installed, and – as I mentioned – things had been working fine since I had installed it… but something had happened in the background which had broken things, and turning off Jetpack Boost as a first test instantly solved the problem.