Steps to Reproduce:

1. Take the latest JIRA Cloud backup.
2. Install JIRA 6.4.6.
3. Put the Cloud backup on the JIRA Home > import
4. Run the Setup Wizard.
5. Choose Import Data
6. Put the Backup name on the File Name

Expected Result:

• There will be a warning, but there is an option that able to be clicked to proceed the restore.

Actual Result:

• The warning below appears:
  

Workaround
Try restoring to a slightly older JIRA server version e.g. 6.4.5 if possible.

NOTES:

• JIRA 6.4 also have the same problem.