I had the same problem. I didn't figure out the cause of this issue, but found a very simple solution. The credentials had expired, so I needed to go to Settings > Online Accounts and select my Google account and log in again. After a few minutes the back-up restarted automatically and finished successfully.
I had the same problem. I didn't figure out the cause of this issue, but found a very simple solution. The credentials had expired, so I needed to go to Settings > Online Accounts and select my Google account and log in again. After a few minutes the back-up restarted automatically and finished successfully.