It is basically only missing a null pointer check. See trace

org.eclipse.core.runtime.AssertionFailedException: null argument:
at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:85)
at org.eclipse.core.runtime.Assert.isNotNull(Assert.java:73)
at org.eclipse.mylyn.tasks.core.data.TaskAttribute.setValue(TaskAttribute.java:617)
at org.eclipse.mylyn.tasks.core.data.TaskAttributeMapper.setValue(TaskAttributeMapper.java:334)
at org.tuleap.mylyn.task.core.internal.repository.TuleapAttributeMapper.setValue(TuleapAttributeMapper.java:57)
at org.eclipse.mylyn.tasks.core.data.TaskAttributeMapper.setRepositoryPerson(TaskAttributeMapper.java:320)
at org.eclipse.mylyn.tasks.core.data.TaskCommentMapper.applyTo(TaskCommentMapper.java:158)
at org.tuleap.mylyn.task.core.internal.data.TuleapArtifactMapper.addComment(TuleapArtifactMapper.java:524)
at org.tuleap.mylyn.task.core.internal.data.converter.ArtifactTaskDataConverter.populateTaskDataConfigurableFields(ArtifactTaskDataConverter.java:163)
at org.tuleap.mylyn.task.core.internal.data.converter.ArtifactTaskDataConverter.populateTaskData(ArtifactTaskDataConverter.java:222)
at org.tuleap.mylyn.task.core.internal.repository.TuleapRepositoryConnector.performReportOrCustomQuery(TuleapRepositoryConnector.java:282)
at org.tuleap.mylyn.task.core.internal.repository.TuleapRepositoryConnector.performQuery(TuleapRepositoryConnector.java:218)
at org.eclipse.mylyn.internal.tasks.core.sync.SynchronizeQueriesJob.synchronizeQuery(SynchronizeQueriesJob.java:312)
at org.eclipse.mylyn.internal.tasks.core.sync.SynchronizeQueriesJob.synchronizeQueries(SynchronizeQueriesJob.java:269)
at org.eclipse.mylyn.internal.tasks.core.sync.SynchronizeQueriesJob.run(SynchronizeQueriesJob.java:201)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)