Filevault and Time Machine blocked

I have recently upgraded a 2011 MBA to High Sierra which converted to APFS. I then enabled Filevault for both users fine; both users are Admins.

I then noticed Time Machine no longer works with either Time Capsule or a USB drive, reporting: "Time Machine did not back up because FileVault was encrypting or decrypting a disk. Backups will resume once FileVault finishes." And this is after I had cleared down my old backups to start again, so I am left with no backup at this time, although my personal data is all safe in a (non apple) cloud service too - just not the installed applications and config.

I then confirmed through all the usual methods that FileVault reports it HAS completed encryption and is not paused or waiting. In doing so I also noticed that the Disable Filevault button is greyed out for both admin users, although I confirmed we both have the SecureTokenOn set in Sysadminctl in Terminal. I also have NO listed option to Turn Off Encryption in the Cmd-R Recovery Disk Utility...unlike older recovery utility versions.

Can anyone now help me, as FDE is less important then making a Time Machine backup, and I have resorted to trying a Terminal session to disable FileVault, but unsuccessfully as I get this response in Terminal (as sudo su):

Last login: Fri Dec 29 09:13:38 on console

Richards-MacBook-Air:~ Richard$ sudo su

Password:

sh-3.2# fdesetup status

FileVault is On.

sh-3.2# fdesetup showdeferralinfo

Not found.

sh-3.2# fdesetup version

fdesetup: Version 6.24

sh-3.2# fdesetup isactive

true

sh-3.2# fdesetup status -extended

FileVault is On.

Volume is APFS. (FileVault Enabled)

sh-3.2# fdesetup disable -verbose

fdesetup: device path = /

Enter the user name:Richard

Enter the password for user 'Richard':

FileVault was not disabled (-69595).

sh-3.2# sudo fdesetup disable -verbose

fdesetup: device path = /

Enter the user name:Richard

Enter the password for user 'Richard':

FileVault was not disabled (-69595).

sh-3.2#

Many thanks for any ideas please, as I am stuck with no backup and unable to disable FDE...urghh!