A quick check of the error log told me what my culprit was. I needed to remove the following line from wp-settings.php:

// For an advanced caching plugin to use, static because you would only want one

strangely enough, I don’t have an advanced caching plugin – though, I think I may have some time ago…but removed it at some point. I removed that line and all was fine after that.

Anyways – another point to provide in the list of information is any errors showing in the error log – - most often that will offer up a clue or two.