I’d tried, and failed, several times to install the Yet Another Related Posts Plugin for WordPress. This time, I actually dug around the forums a bit to see why it wasn’t working, and now it’s fixed.
YARPP requires MySQL version 4.1 or later. Partly due to the age of my blog, I still had my WordPress database at 1&1 at MySQL 4.0. They now offer MySQL 5, and any new databases are created under the newer version. As I’ve used less than 5% of my database allocation, it was a simple job to create a new database, backup the old one, restore it to the new database, then point my wp-config.php to the new DB.
(If it still doesn’t work, view the source to your Settings→YARPP configuration page. Error messages are helpfully, if cryptically, embedded in HTML comments.)
I’ll probably still get a few “No Related Posts” appearing until the cache fills, but that should go away soon.