Install Lubuntu on an encrypted partition with UEFI as boot mode.
The installer (calamares) creates an encrypted LUKS container for /, an unencrypted partition for /boot and Grub is installed on the EFI-System-Partition.
The separate /boot-partition is not created.
Booting the installed system does not work, because of missing Grub modules on the EFI-System-Partition.
See the upstream issue => Calamares Issue 1083
Possible workaround (untested):
If I remember well, after installation you have to boot the live system, open the LUKS container and copy the /boot/grub/x86_64-efi folder on your EFI-System-Partition (EFI/ubuntu/x86_64-efi).
After this, Grub should find the needed Grub modules to boot the installed system.