Why have I got «no matching swap device is available» on `update-initramfs -u`?

This message indicates that the initramfs configuration (usually the file /etc/initramfs-tools/conf.d/resume) specifies the UUID of a swap partition from which the system expects to hibernate/resume.
However, when creating a new initrd (via update-initramfs -u), this partition was not found under the specified UUID, which is why the scripts complain: «no matching swap device is available».