I installed Kubuntu 20.04 on a completely encrypted disk / partition. Now I would like to shrink the encrypted partition to backup it. I tried to do it with a Live USB Stick with GParted 1.1.0 (one of the newest versions that can handle LVM / encryption). I read that old GParted version can't do it, but this one is a new one. I can unlock the partition, but it is not possible to resize it. Why? How can I do it?
SnapOverflow
You should be able to do it with KDE Partition Manager from LiveUSB. What problem did you hit.
To resize encrypted partition you need to first unlock it. Resizing encrypted contents might need contents to be unmounted (e.g. if you use ext4), hence Live USB. Some filesystems (btrfs) an be resized without unmounting them, in that case you don't even need to boot LiveUSB and can do from the running system.
If you use LVM together with LUKS, you first need to resize LVM logical volumes (which KDE Partition Manager can also do).