LoadRunner logged an error " Failed adding AssertionItem"

Hello,
we get an error message when a load test is running:

13:37:26,126 ERROR [AssertionAddonImpl] Failed adding AssertionItem
java.lang.NullPointerException: Cannot invoke "com.eviware.loadui.api.statistics.StatisticVariable.getStatistic(String, String)" because the return value of "com.eviware.loadui.api.statistics.StatisticHolder.getStatisticVariable(String)" is null
	at com.eviware.loadui.util.serialization.StatisticResolver.getValue(StatisticResolver.java:62) ~[loadui-utils-3.30.0.jar:?]
	at com.eviware.loadui.util.serialization.StatisticResolver.getValue(StatisticResolver.java:1) ~[loadui-utils-3.30.0.jar:?]
	at com.eviware.loadui.impl.assertion.AssertionItemImpl.listenableValueFrom(AssertionItemImpl.java:210) ~[loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.assertion.AssertionItemImpl.<init>(AssertionItemImpl.java:186) ~[loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.assertion.AssertionAddonImpl.initialize(AssertionAddonImpl.java:116) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.assertion.AssertionAddonImpl.<init>(AssertionAddonImpl.java:106) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.factory.AddonFactoryImpl.createAssertionAddon(AddonFactoryImpl.java:22) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.assertion.AssertionAddonImpl$Factory.create(AssertionAddonImpl.java:279) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.assertion.AssertionAddonImpl$Factory.create(AssertionAddonImpl.java:1) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.addon.AddonHolderSupportImpl.ensureAddon(AddonHolderSupportImpl.java:94) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.addon.AddonHolderSupportImpl.getAddon(AddonHolderSupportImpl.java:66) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.ModelItemImpl.getAddon(ModelItemImpl.java:184) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.addon.AddonRegistryImpl.loadAddon(AddonRegistryImpl.java:51) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.addon.AddonRegistryImpl.registerAddonHolder(AddonRegistryImpl.java:114) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.addon.AddonHolderSupportImpl.init(AddonHolderSupportImpl.java:60) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.ModelItemImpl.postInit(ModelItemImpl.java:109) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.canvas.SceneItemImpl.postInit(SceneItemImpl.java:163) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.canvas.SceneItemImpl.newInstance(SceneItemImpl.java:146) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.canvas.project.ProjectItemImpl.loadScenes(ProjectItemImpl.java:366) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.canvas.project.ProjectItemImpl.init(ProjectItemImpl.java:298) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.canvas.project.ProjectItemImpl.superInit(ProjectItemImpl.java:288) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.model.canvas.project.ProjectItemImpl.<init>(ProjectItemImpl.java:200) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.factory.LoadUIModelItemFactoryImpl.createProjectItem(LoadUIModelItemFactoryImpl.java:72) [loadui-base-3.30.0.jar:?]
	at com.smartbear.load.impl.LoadUITestImpl.initProjectItem(LoadUITestImpl.java:108) [loadui-base-3.30.0.jar:?]
	at com.smartbear.load.impl.LoadUITestImpl.getProjectItem(LoadUITestImpl.java:97) [loadui-base-3.30.0.jar:?]
	at com.smartbear.load.impl.LoadUITestImpl.<init>(LoadUITestImpl.java:88) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.impl.factory.LoadUIModelItemFactoryImpl.createLoadUITest(LoadUIModelItemFactoryImpl.java:52) [loadui-base-3.30.0.jar:?]
	at com.eviware.loadui.cmd.LoadUITestCmdLineRunner.getTest(LoadUITestCmdLineRunner.java:741) [loadui-cmd-base-3.30.0.jar:?]
	at com.eviware.loadui.cmd.LoadUITestCmdLineRunner.start(LoadUITestCmdLineRunner.java:150) [loadui-cmd-base-3.30.0.jar:?]
	at com.smartbear.ready.cmd.runner.loaduicommandlinerunner.LoadUIProjectRunner.runRunner(LoadUIProjectRunner.java:166) [ready-api-runners-3.30.0.jar:?]
	at com.smartbear.ready.cmd.runner.AbstractSoapUIRunner.run(AbstractSoapUIRunner.java:333) [ready-api-runners-3.30.0.jar:?]
	at com.smartbear.ready.cmd.runner.AbstractSoapUIRunner.runFromCommandLine(AbstractSoapUIRunner.java:198) [ready-api-runners-3.30.0.jar:?]
	at com.smartbear.ready.cmd.runner.loaduicommandlinerunner.LoadUIProjectRunner.main(LoadUIProjectRunner.java:63) [ready-api-runners-3.30.0.jar:?]

We used ReadyAPI 3.10.x, 3.20.x and 3.30.0.
Know somebody why the errors will be logged?