If you're experiencing this error, we apologise for the inconvenience.
This happens due to an error coming from your Google Drive automatic backups. The one you got most commonly means that we've run out of space on Google Drive (but other times is just a generic failure on their end).
If you launch Google Drive (https://drive.google.com/drive/my-drive), you can check the storage usage on the mid-to-bottom left.
Alternatively, here's an article about it: https://ihowpc.com/check-google-drive-storage-space
If you have plenty of space left, we've also seen this occur due to a transient Google Drive error that resolves itself after some time. Google doesn't have separate errors for lack of space vs the transient failures, so we err on the side of caution with our messaging and alert either way.