I created backup of Confluence Cloud using Backup manager (button "Create backup for cloud"), download it and tried to restore original file (Import spaces menu, selected original zip archive).
Restoring fails right after unzipping on server with error: «Only Confluence space exports can be imported». Again and again.

I unsuccessfully tried to unzip backup and zip it using Terminal to exclude macOS files (.DS_Store and etc). The same error.
So, I cleaned Confluence Cloud and created new backup with only one space and one page and tried to restore it on macOS and Windows 10. Both times it failed.
Looks like restoring backups not working…
How can I restore my data from Confluence Cloud backup?
How can I know why restoring backup fails, can I see logs of Confluence Cloud?
What are the requirements to backup file, why it can’t be restored?