SUMMARY
Using the widget to create a new vault gives error "Unable to perform the operation (error code 1)." and then subsequently opening the vault opens the mount point unencrypted.

STEPS TO REPRODUCE
1. +Create a New Vault...
2. Vault name: MyVault
3. Backend: CryFS (i.e. leave at default setting)
4. Enter and verify password
5. Encrypted data location: /home/paul/.local/share/plasma-vault/MyVault.enc (i.e. the default)
6. Mount point: /home/paul/Vaults/MyVault (i.e. the default)
7. Use the default cipher
8. NOT Limit to selected activities
9. NOT Go offline
10. Click "Create"

OBSERVED RESULT
"Unable to perform the operation (error code 1)."
Dialog stays open
No entry for MyVault in the widget's list of vaults

EXPECTED RESULT
No error msg
Vault is created
Vault appears in widget list

SOFTWARE VERSIONS
Kubuntu 18.10
KDE Plasma Version: 5.14.2 installed via backports PPA
KDE Frameworks Version: 5.51.0
Qt Version: 5.11.1

ADDITIONAL INFORMATION
/home is NFS4 mounted from a server on my private network. (would this make a difference?)

Using the widget as above: 

A new folder "/home/Vaults/MyVault" is created

A new folder "/home/.local/share/plasma-vault/MyVault.enc" is created

/home/.config/plasmavaultrc is updated with:

[/home/paul/.local/share/plasma-vault/MyVault.enc]
activities=
backend=cryfs
lastError=Unknown device (code: 1)
lastStatus=0
mountPoint=/home/paul/Vaults/MyVault
name=MyVault
offlineOnly=false

Next time I log in, MyVault appears in Widget list as a normal folder but the 'mount' button does nothing.

If I choose "Open with File Manager", and enter my password the Vault appears to be opened in Dolphin, but it's actually the unencrypted mount point. (no cryfs filesystem is reported by the 'mount' command).

If I use the widget to delete MyVault and then on the command-line:

cryfs /home/paul/.local/share/plasma-vault/MyVault.enc /home/paul/Vaults/MyVault

and choosing default settings give no error message and basedir is mounted successfully.