I have installed Xubuntu 20.04 encrypted on an SSD and get the following error on boot time:
/tmp/mountroot-fail-hooks.d//scripts/init-premount/lvm2: No such file or directory
The boot time is pretty slow, so I think that could be one of the reasons.
Is the reason for the error the double slash after hooks.d?
In my case, that was due to a misconfiguration into /etc/fstab I previously installed Ubuntu with an encrypted HDD (LVM2) that i decided to remove (I removed the encryption and the initial partitions, not the HDD).
I had exactly the same problem (slow boot and same message). I just removed entries from /etc/fstab about my previous partitions (that was also using mapper). And that rules.
Boot is as quick as intended now. And I modified /etc/fstab to consider my new "normal" partitions.
I bet your message and the slowness is due to a misconfiguration into /etc/fstab trying to load wrong partition.
Hope that helped.