If you create a vault named for example "PrivateVault" using cryfs algo, then remove it, so any attempt to create a new vault with the same name "PrivateVault" and same encryption location will fail with error code:
"Unable to perform the operation (error code 20)."
The problem is that after successfully deleting that vault, some traces still persist in "/home/user/.config/plasmavaultrc" and "/home/user/.local/share/cryfs/". Removing those traces will solve the problem.

Another problem is that if creation operation fails you will find some junk files in encryption location like "cryfs.config" and the mount target is created and some entries are added to "plasmavaultrc". "cryfs.config" file and the target mount location if they don't exist before should never be created if the creation operation fails.