SUMMARY STEPS TO REPRODUCE 1. Setup LVM or Luks Encrypted parition with home and root inside it. 2. Create an unencrypted boot partition 3. Run throught the installer and select manual paritioning 4. Assign the partitions and then click install 5. Fill in user details and click continue 6. The installer will crash either because you choose to format a parittion or it will fail when the installer tries to install grub. OBSERVED RESULT Installer crashes either trying to format paritions or when the install tries to install grub. The target parition will be empty after the installer crashes. EXPECTED RESULT Install of KDE Neon and for the installer not to crash. SOFTWARE/OS VERSIONS KDE Neon: 20.04 Notes: Sad to see the use of the calamares which seems completely unstable with no benefits. Doesn't seem to cope with custom setups at all. The old installer was generally rock solid. I think while migrating providing the old installer along side would have been a sensible move.
I installed yesterday with a manually created btrfs (which had to be formatted), I re-used the previous boot partition, I had no problems so maybe something more specific to your problem rather than "Doesn't seem to cope with custom setups at all". Calamares is indeed a WIP but as more and more work goes into it the stability is growing.
You'll generally want to file partitioning problems with calamares.