Summary,

• Due to some changes in the latest versions of Confluence, it is not possible to import, in some cases, from later to earlier versions. This issue is when attachments are included.

How to replicate

• Create a space and export with attachments (later major version)
• Import the space (earlier major version)

Expected results

• The space is imported successfully

Actual results:

• Throws the following error:

  Import failed. Check your server logs for more information. com.atlassian.confluence.importexport.ImportExportException: Unable to complete import: Error while importing backup: not-null property references a null or transient value: com.atlassian.confluence.pages.Attachment.contentType

Investigation done

• I assume it fails because of attachments table was dropped in later versions.

Workaround

• Backup without the attachments
• From earlier to later versions works with or without attachments