The following is simply not enough to figure out what is wrong with a report file:

[xUnit] [INFO] - Starting to record.
[xUnit] [INFO] - Processing JUnit
[xUnit] [INFO] - [JUnit] - 1 test report file(s) were found with the pattern '**/valgrind-junit.xml' relative to '/home/jenkins/workspace/workspace/valgrind' for the testing framework 'JUnit'.
[xUnit] [ERROR] - The result file '/home/jenkins/workspace/workspace/valgrind/valgrind-junit.xml' for the metric 'JUnit' is not valid. The result file has been skipped.
[xUnit] [INFO] - Setting the build status to FAILURE
[xUnit] [INFO] - Stopping recording.

It really needs to say at least what line caused the problem, and preferably what it thought the problem was.