RS sort issue in page list of portfolio

RedScreen on hfgs 11.2.1
2017-02-07 21:36:26,890 [ajp-nio2-8004-exec-31] ERROR OLATRuntimeException - OLAT::ERROR ^%^ N4-E24 ^%^ org.olat.core.logging 

https://e.xyz.ch/auth/HomeSite/280035342/PortfolioV2/0/MyBinders/0/Binder/49/Toc/0 ^%^ Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36 

**RedScreen** Comparison method violates its general contract!

<dispatchinfo>
	<componentinfo>
		<compname>portfolio.entries</compname>
		<compclass>org.olat.core.gui.components.link.Link</compclass>
		<extendedinfo>n/a</extendedinfo>
		<event>
			<class&gt;org.olat.core.gui.control.Event</class&gt;
			<command>portfolio.entries</command>
			<tostring>com:portfolio.entries,org.olat.core.gui.control.Event@6adfd209</tostring>
		</event>
	</componentinfo>
	<controllerinfo>
		<controllername>org.olat.modules.portfolio.ui.BinderController</controllername>
		<controllername>org.olat.modules.portfolio.ui.BinderListController</controllername>
		<controllername>org.olat.modules.portfolio.ui.PortfolioHomeController</controllername>
		<controllername>org.olat.modules.portfolio.ui.PortfolioPersonalToolController</controllername>
		<controllername>org.olat.home.HomeMainController</controllername>
		<controllername>org.olat.core.commons.fullWebApp.BaseFullWebappController</controllername>
	</controllerinfo>
</dispatchinfo>

>>> stack of 1.cause::org.olat.core.logging.OLATRuntimeException: Comparison method violates its general contract!
-> at org.olat.core.gui.exception.ExceptionWindowController.<init>(ExceptionWindowController.java:116)
   at org.olat.core.gui.exception.MsgFactory.createMessageChiefController(MsgFactory.java:57)
   at org.olat.core.gui.exception.MsgFactory.createMessageChiefController(MsgFactory.java:46)
   at org.olat.core.gui.components.Window.dispatchRequest(Window.java:548)
   at org.olat.core.gui.components.Window.doDispatchRequest(Window.java:241)
   at org.olat.core.gui.components.AbstractComponent.dispatchRequest(AbstractComponent.java:193)
   at org.olat.dispatcher.AuthenticatedDispatcher.processValidDispatchURI(AuthenticatedDispatcher.java:273)
   at org.olat.dispatcher.AuthenticatedDispatcher.execute(AuthenticatedDispatcher.java:190)
   at org.olat.core.servlets.OpenOLATServlet.executeUserRequest(OpenOLATServlet.java:292)
   at org.olat.core.servlets.OpenOLATServlet.doPost(OpenOLATServlet.java:235)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
>>> stack of 2.cause::java.lang.IllegalArgumentException: Comparison method violates its general contract!
-> at java.util.TimSort.mergeHi(TimSort.java:899)
   at java.util.TimSort.mergeAt(TimSort.java:516)
   at java.util.TimSort.mergeForceCollapse(TimSort.java:457)
   at java.util.TimSort.sort(TimSort.java:254)
   at java.util.Arrays.sort(Arrays.java:1512)
   at java.util.ArrayList.sort(ArrayList.java:1454)
   at java.util.Collections.sort(Collections.java:175)
   at org.olat.modules.portfolio.ui.PageListSortableDataModelDelegate.sort(PageListSortableDataModelDelegate.java:66)
   at org.olat.core.gui.components.form.flexible.impl.elements.table.SortableFlexiTableModelDelegate.sort(SortableFlexiTableModelDelegate.java:95)
   at org.olat.modules.portfolio.ui.PageListDataModel.sort(PageListDataModel.java:54)
   at org.olat.core.gui.components.form.flexible.impl.elements.table.FlexiTableElementImpl.reloadData(FlexiTableElementImpl.java:1582)

2017-02-14 21:59:23,190 [ajp-nio2-8004-exec-2] ERROR PageListDataModel - OLAT::ERROR ^%^ N4-E7 ^%^ org.olat.modules.portfolio.ui 
https://e.xyz.ch/auth/HomeSite/280035342/PortfolioV2/0/MyBinders/0/Binder/49/Entries/0/Entry/1474 ^%^ Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36
Cannot sort with: sort[key=null:asc=false]
>>>stack of 1.cause::java.lang.IllegalArgumentException: Comparison method violates its general contract!
-> at java.util.TimSort.mergeHi(TimSort.java:899)
   at java.util.TimSort.mergeAt(TimSort.java:516)
   at java.util.TimSort.mergeCollapse(TimSort.java:441)
   at java.util.TimSort.sort(TimSort.java:245)
   at java.util.Arrays.sort(Arrays.java:1512)
   at java.util.ArrayList.sort(ArrayList.java:1454)
   at java.util.Collections.sort(Collections.java:175)
   at org.olat.modules.portfolio.ui.PageListSortableDataModelDelegate.sort(PageListSortableDataModelDelegate.java:66)
   at org.o[...]