ERROR: Cookies are blocked or not supported by your browser

I’m getting this error while trying to log in:

ERROR: Cookies are blocked or not supported by your browser. You must
enable cookies to use WordPress.

Yet I have cookies enabled and not blocked. I’ve tried this on a few different browsers, and in the incognito/private modes of those browsers, too, and get the same messages. I’ve also tried clearing all cookies from the cache, pursuant to the instructions on Mozilla Support. What could be causing this, and how would I go about debugging this issue?

It turns out I had DOMAIN_CURRENT_SITE set incorrectly in wp-config.php. (This particular install was a copy of another server, and the domain hadn’t fully been changed.)