The following Null Pointer Exception occurs when the Testlink Plugin parses a JUnit test result file.
This JUnit file is generated by QF Test (GUI Test Tool). See attachment.

Here the Console output:

...

Looking for the test results of TestLink test cases.

Found [18] JUnit XML(s). Parsing JUnit XML [MST_DEMO_LOG_01/MST_DEMO_LOG_01_201105120834/MST_DEMO_LOG_01_report_junit.xml]. Veryfying JUnit test suite [/data/tester/system_tests/qftest/demos/MST_DEMO_LOG_01/MST_DEMO_LOG_01_qft]. This suite contains [3] tests, [0] failures and [0] errors.

Verifying JUnit test [UnitTest.Startup].
Looking for a test result that matches with key custom field [TESTCLASS] => [null].
Verifying TestLink test case [MST_DEMO_LOG_01], ID [111].
Custom fields in this TestLink test case [[CustomField [id=14, name=TESTCLASS, label=TESTCLASS, type=0, possibleValues=, defaultValue=, validRegexp=, lengthMin=0, lengthMax=0, showOnDesign=true, enableOnDesign=true, showOnExecution=true, enableOnExecution=false, showOnTestPlanDesign=false, enableOnTestPlanDesign=false, displayOrder=1, location=1, value=MST_DEMO_LOG_01], CustomField [id=12, name=test_suite, label=Test Suite, type=0, possibleValues=, defaultValue=, validRegexp=, lengthMin=0, lengthMax=0, showOnDesign=true, enableOnDesign=true, showOnExecution=true, enableOnExecution=false, showOnTestPlanDesign=false, enableOnTestPlanDesign=false, displayOrder=1, location=1, value=/data/tester/system_tests/qftest/demos/MST_DEMO_LOG_01/MST_DEMO_LOG_01.qft], CustomField [id=9, name=automation_type, label=Automation Type, type=6, possibleValues=QFTest|TADS, defaultValue=, validRegexp=, lengthMin=0, lengthMax=0, showOnDesign=true, enableOnDesign=true, showOnExecution=true, enableOnExecution=false, showOnTestPlanDesign=false, enableOnTestPlanDesign=false, displayOrder=1, location=1, value=QFTest], CustomField [id=15, name=work_dir, label=Working Directory, type=0, possibleValues=, defaultValue=, validRegexp=, lengthMin=0, lengthMax=0, showOnDesign=true, enableOnDesign=true, showOnExecution=true, enableOnExecution=false, showOnTestPlanDesign=false, enableOnTestPlanDesign=false, displayOrder=1, location=1, value=/data/tester/system_tests/qftest/demos/MST_DEMO_LOG_01/], CustomField [id=10, name=execution_host, label=Execution Host, type=0, possibleValues=, defaultValue=, validRegexp=, lengthMin=0, lengthMax=0, showOnDesign=true, enableOnDesign=true, showOnExecution=true, enableOnExecution=false, showOnTestPlanDesign=false, enableOnTestPlanDesign=false, displayOrder=2, location=1, value=mclient35]]].
An error occured while trying to retrieve the test results: Unkown internal error. Please, open an issue in Jenkins JIRA with the complete stack trace. If possible, try including the JUnit XML(s) too..
FATAL: Unkown internal error. Please, open an issue in Jenkins JIRA with the complete stack trace. If possible, try including the JUnit XML(s) too.
hudson.plugins.testlink.result.TestResultSeekerException: Unkown internal error. Please, open an issue in Jenkins JIRA with the complete stack trace. If possible, try including the JUnit XML(s) too.
at hudson.plugins.testlink.result.JUnitTestResultSeeker.seek(JUnitTestResultSeeker.java:113)
at hudson.plugins.testlink.result.TestResultsCallable.seekTestResults(TestResultsCallable.java:115)
at hudson.plugins.testlink.result.TestResultsCallable.invoke(TestResultsCallable.java:156)
at hudson.plugins.testlink.result.TestResultsCallable.invoke(TestResultsCallable.java:45)
at hudson.FilePath.act(FilePath.java:757)
at hudson.FilePath.act(FilePath.java:739)
at hudson.plugins.testlink.TestLinkBuilder.perform(TestLinkBuilder.java:503)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:644)
at hudson.model.Build$RunnerImpl.build(Build.java:175)
at hudson.model.Build$RunnerImpl.doRun(Build.java:137)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:421)
at hudson.model.Run.run(Run.java:1362)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:145)
Caused by: java.lang.NullPointerException
at hudson.plugins.testlink.result.JUnitTestResultSeeker.doFindTestResult(JUnitTestResultSeeker.java:246)
at hudson.plugins.testlink.result.JUnitTestResultSeeker.doJunitSuite(JUnitTestResultSeeker.java:180)
at hudson.plugins.testlink.result.JUnitTestResultSeeker.doJunitReports(JUnitTestResultSeeker.java:145)
at hudson.plugins.testlink.result.JUnitTestResultSeeker.seek(JUnitTestResultSeeker.java:105)
... 15 more

FATAL: null
java.lang.NullPointerException
at hudson.plugins.testlink.TestLinkService.updateTestCasesAndUploadAttachments(TestLinkService.java:155)
at hudson.plugins.testlink.TestLinkBuilder.perform(TestLinkBuilder.java:525)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:644)
at hudson.model.Build$RunnerImpl.build(Build.java:175)
at hudson.model.Build$RunnerImpl.doRun(Build.java:137)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:421)
at hudson.model.Run.run(Run.java:1362)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:145)