I just tried to reproduce the problem, but it worked fine for me. Here is what I did: - Fresh installation with a plain partition (no LVM) ext4 root - Created an encrypted ext4 partition as /data - Executed installation - New installation - Entered password for the encrypted partition when prompted - In the storage proposal, changed proposal settings to plain partition ext4 as root - Went into the expert partitioner - Selected that encrypted partition, clicked "edit", then "do not format", "mount at /data" - Confirmed and performed installation -> no error, worked as expected.