I see this error all the time as well.
BUT… this error happens after the restore, and it finishes with SUCCESS. So it is an error, but it is certainly not causing restores to fail.
Unpausing sidekiq...
Marking restore as finished...
Notifying 'DHSupport' of the end of the restore...
Something went wrong while notifying user.
Finished!
[SUCCESS]
Are you sure there is not an error higher up?