iCloud Backup failing with CloudKit errors

My iCloud Backups have been failing since 7th May with the error "The last backup could not be completed".

This is an iPhone 6S running iOS 9.3.2. I have deleted old backups, turned iCloud Backups off/on and restarted my iPhone numerous times but backups still consistently fail. I have 50GB of iCloud storage and over 20GB available - the backups were previously only consuming about 3-4GB. I have 15GB of available storage on my iPhone itself. The backups run for about 2 hours before failing. I have a high-speed internet connection and do not suspect network errors.

In an effort to diagnose this issue I connected my iPhone to my Mac and captured the below console messages from iOS.

This shows CloudKit reporting errors that cause the backup to be marked as failed.

Has anyone experienced similar or is anyone able to help troubleshoot what might be causing this issue?

Thanks

May 25 11:24:24 backupd[420] <Warning>: ERROR: Failed to disable backups

May 25 11:24:24 backupd[420] <Warning>: WARNING: Files modified during backup

May 25 11:24:24 backupd[420] <Warning>: INFO: Retrying after soft error: Saving asset failed at path "/var/tmp/tmp.531.J8nkLZ" (CKErrorDomain/4). Underlying error: Saving asset failed at path "/var/tmp/tmp.531.J8nkLZ" (CKInternalErrorDomain/1013). Underlying error: The operation couldn’t be completed. (com.apple.mmcs error 7.) (com.apple.mmcs/7). Underlying error: The operation couldn’t be completed. (com.apple.mmcs error 12.) (com.apple.mmcs/12)...

May 25 11:24:34 backupd[420] <Warning>: WARNING: Retrying after error: <CKError 0x1273e8770: "Network Failure" (4/1013); "Saving asset failed">

May 25 11:24:34 backupd[420] <Warning>: INFO: Reclaiming quota from pending items for domain: HomeDomain

May 25 11:24:35 backupd[420] <Warning>: INFO: Uploading files for domain: HomeDomain

May 25 11:24:35 backupd[420] <Error>: MKBBackupValidateKey: Key blob is for a file with a different inode (101613 instead of 19145054)

May 25 11:24:35 backupd[420] <Error>: MKBBackupValidateKey: Key blob is for a file with a different inode (16788076 instead of 19145063)

May 25 11:24:37 backupd[420] <Error>: MKBBackupValidateKey: Key blob is for a file with a different inode (99197 instead of 19145156)

May 25 11:29:31 backupd[420] <Warning>: WARNING: Files modified during backup

May 25 11:29:31 backupd[420] <Warning>: INFO: Not retrying after soft error: Saving asset failed at path "/var/tmp/tmp.571.zvPTRQ" (CKErrorDomain/6). Underlying error: Saving asset failed at path "/var/tmp/tmp.571.zvPTRQ" (CKInternalErrorDomain/2022). Underlying error: The operation couldn’t be completed. (com.apple.mmcs error 16.) (com.apple.mmcs/16)..

May 25 11:29:31 backupd[420] <Warning>: ERROR: Backup failed: <CKError 0x125dcb650: "Service Unavailable" (6/2022); "Saving asset failed"; Retry after 3.0 seconds>

May 25 11:29:31 backupd[420] <Warning>: ERROR: ========== Backup failed: Saving asset failed at path "/var/tmp/tmp.571.zvPTRQ" (CKErrorDomain/6). Underlying error: Saving asset failed at path "/var/tmp/tmp.571.zvPTRQ" (CKInternalErrorDomain/2022). Underlying error: The operation couldn’t be completed. (com.apple.mmcs error 16.) (com.apple.mmcs/16)..