In the actual build itself (${JOB_WORKSPACE} is valid):

[xUnit] [INFO] - Starting to record.
[xUnit] [INFO] - Processing JUnit
[xUnit] [INFO] - [JUnit] - 1 test report file(s) were found with the pattern 'junit.xml' relative to '${JOB_WORKSPACE}' for the testing framework 'JUnit'.
[xUnit] [ERROR] - There is at least one problem. Check the Jenkins system log for more information. (if you don't have configured yet the system log before, you have to rebuild).
[xUnit] [INFO] - There are errors when processing test results.
[xUnit] [INFO] - Skipping tests recording.
Performance: Percentage of errors greater or equal than 0% sets the build as unstable
Performance: Percentage of errors greater or equal than 0% sets the build as failure
Performance: Recording JUnit reports 'junit.xml'
Performance: Parsing JUnit report file junit.xml
Performance: File junit.xml reported 0.4618937644341801% of errors [FAILURE]. Build status is: FAILURE

I get this in my system logs:

Sep 27, 2013 10:59:05 AM WARNING com.thalesgroup.hudson.plugins.xunit.service.XUnitService warningSystemLogger
[xUnit] - The file 'junit.xml' is an invalid file.
Sep 27, 2013 10:59:05 AM SEVERE com.thalesgroup.hudson.plugins.xunit.service.XUnitService errorSystemLogger
[xUnit] - The result file 'junit.xml' for the metric 'JUnit' is not valid. The result file has been skipped.

It does not make sense to me why it would fail for xUnit but not performance.