On one laptop I'm running the previous KDE neon version based on 18.04 and I could easily setup the following partitions
sda 8:0 0 238.5G 0 disk
├─sda1 8:1 0 1G 0 part /boot
└─sda2 8:2 0 237.5G 0 part
└─CryptDisk 253:0 0 237.5G 0 crypt
├─vg0-swap 253:1 0 16G 0 lvm [SWAP]
├─vg0-root 253:2 0 40G 0 lvm /
└─vg0-home 253:3 0 181.5G 0 lvm /home

On the other laptop I tried to install the new version but the installer does not accept a partitioning schema like the one above.
If you create the Phisical Volume, Volume Group and Logical Volumes via CLI and then switch to the installer it always tries to delete the VG and hangs showing the old partition scheme.
If you choose the default partitioning with encryption it does not even split root from home and creates something like this:
sda 8:0 0 465.8G 0 disk
├─sda1 8:1 0 300M 0 part /boot/efi
└─sda2 8:2 0 457.3G 0 part
| └─luks-<blk-id> 253:0 0 457.3G 0 crypt /
└─sda3 8:2 0 8.2G 0 part
└─luks-<blk-id> 253:1 0 8.2G 0 crypt [SWAP]

Any idea how to fix this?

Did you create a valid file system on the logical volumes?