I moved a brand new WordPress site on to new hosting recently and was confronted by an “Updating failed. Error message: The response is not a valid JSON response” error. Seemed kind of inexplicable, not a lot of info in the console either. I’ve done the same thing a bajillion times with this and other hosting providers and have never run in to this error, so it seems kind of weird.
This issue on GitHub outlines a lot of the potential causes, but this comment specifically sorted out my problem. Turns out you just need to flush the permalinks? Another off-then-on-again type of fix.
I think the WordPress devs might eventually create a more helpful error message for this, but in the meantime this is worth keeping in mind.