Thank you for your reply! It appears the issue was that the update deleted a huge section of pluggable.php, which may relate to that shortcode link. Either way, it’s something I’ll bear in mind if any other issues happen after this upgrade. I restored the original pluggable file and it’s working fine now.
If anyone else is having this issue, apparently the updating may not yet be complete, so it’s worth waiting until tomorrow before blindly panicking (like I did).
