If you have the jobs organized in folders (without any job in the first level) when you try to Export as plain XML in the Build History section, then you will get an empty XML.
Steps to reproduce the issue:
- Create the folder A
- Create a simple freestyle job "test" inside the folder A and build it several times.
- Access to Build History in the left menu and you will see all the builds executed previously
- Click Export as plain XML and you will receive an empty XML.