If you are in need to do xml backup on a Confluence, please note – Atlassian recommend other production backup strategy and don’t recommend to use xml backups for the large sites (2GB or larger) – Backup Strategy
If you need the xml backup – try to increase memory on the host – there is often memory problems when you do such action.
As from our experience it is often impossible to make backup using simply the button , but somehow backup is successful as scheduled job.
Check here for Backup Configuration – set up automatic backups and get one from the export folder.