A customer has recently experienced this bug. The initial problem was that their space export kept failing and after further testing and troubleshooting, it was found out that the deleted content in the trash was at fault. Upon trying to purge the trash through the UI, it failed. The workaround was to create an sql script that cleaned up and deleted everything associated to all of the content that was marked as deleted.
This is apparently a known bug but we were not able to find a bug report for it. Apparently, the TEST space on confluence.atlassian.com also fails to purge.