Do you see any error in the UI or in the atlassian-confluence.log?
See if you can find more clues in your logs, so we can try to better assist you.
I will suggest you to create a support ticket at support.atlassian.com, so you can attach your complete logs and we can provide you a more assertive answer.
To better understand what's avoiding the export to be completed, can you provide us more details of the information tracked in the atlassian-confluence.log file which is inside <Confluence Home/logs> directory, so we can give you a better direction?
PS: To help you in the meantime, you can take a look at this KB article which will guide you with directions according to the issue found. :)
Hope it helps!
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