Ubuntu – Deja-dup repeatedly asks encryption password

You can't easily change the password unfortunately. In order to do so, you must either create a fresh backup in a new location or delete the existing backup files and back up again.

However, it sounds like you may have hit a bug. Please consider filing a bug at https://launchpad.net/deja-dup/+filebug

It seems that you hit a known bug: deja-dup bug 826389, bug 487720.

Quoting from the second link it seems that your data corruption is not given by hardware, but is completely software-related:

This can happen when a volume file was not completely written to the backend before duplicity was interrupted (say, shutting down the machine or whatever). When duplicity resumes the backup next run, it will start with the next volume. The half-complete volume file will sit on the backend and cause this error later when restoring.

You can manually recover from this by either restoring from your older backup sets or by restoring individual files that don't happen to be in the corrupted volume.

Two other guys from the first link says:

There are two ways around this. You can try to restore from the backup from before the corrupted one. So try restoring from older backups.

You can also try to avoid the specific volume by restoring all the files from the backup set except the ones in the corrupted volume.

and:

Thank you very much. What I did was [duplicity --file-to-restore ....] and I restored the important files.

I've never used deja-dup or duplicity, but it seems that the most reasonable solution here is to automatically restore intact volumes with deja-dup, and then proceed with intact files inside a volume restoring them one by one with duplicity --file-to-restore ...

Hope this helps.