The JIRA Cloud Backup Manager is currently throwing the following error when attachments are missing for OnDemand:

In the UI:

Woah! Something went wrong!
Unexpected error during export has occurred

The UI also shows red text on a red background, which needs to be fixed.

In the logs:

com.atlassian.jira.issue.attachment.AttachmentReadException: java.io.FileNotFoundException: /data/jirastudio/jira/home/data/attachments/ABC/ABC-123/10000 (No such file or directory)

Missing attachments is undesirable, but it can happen if the attachments weren't imported for example. It should throw a warning, not an error.

Workaround

Create a backup without attachments, and then log a ticket at https://support.atlassian.com and ask the Support Engineer to provide you with a zip of your attachment directory.