cryptsetup: Cannot allocate memory

[pth3434]

Fresh install on RasPi 3+, set up root filesystem encryption with LVM on LUKS. During boot the passphrase prompt shows up as usual, but the disk fails to unlock. Looking in the logs (or trying to invoke systemd-cryptsetup manually) I get the following error:
$this->bbcode_second_pass_code('', 'Failed to activate: Cannot allocate memory')

However, free reports 800MB+ free memory... huh?

Currently at a loss at how to debug this... Also tried falling back to the udev mkinitcpio hooks, but to no avail.

Posting here instead of on "General" because the same configuration works on my RasPi 2 (armv7h), but doesn't on the Raspi 3+.