Nice! The new Dracut 047 seems to have solved the issue for me. Steps:

I also have /etc/crypttab. I've created by myself during the os installation... But yes, solus-installer does not create it. Anyway I have the error too if I boot with rd.luks.uuid or with rd.luks.crypttab=no.

Hi all! I was annoyed too by the failed to load message on boot. And I finally get it gone. The trick is to replace in the boot line rd.luks.uuid=[...] by luks.uuid=[...]. Don't ask :) it just works. I more or less have an idea of why after reading the man page of systemd-cryptsetup-generator... But I wonder if anyone could give the exact explanation.