XPages: Error while saving document after upgrade to 8.5.2

After a server upgrade to 8.5.2. we experience some problems with an xPage application we developed.

In a specific custom control we added a submit button which executes a simple action to save the current document. After the button is pressed we almost instantly get the following error:

java.util.TreeMap incompatible with java.util.List

javax.faces.FacesException: java.util.TreeMap incompatible with java.util.List

com.sun.faces.lifecycle.ProcessValidationsPhase.execute(ProcessValidationsPhase.java:105)

com.sun.faces.lifecycle.LifecycleImpl.phase(LifecycleImpl.java:210)

com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:96)

com.ibm.xsp.controller.FacesControllerImpl.execute(FacesControllerImpl.java:250)

com.ibm.xsp.webapp.FacesServlet.serviceView(FacesServlet.java:206)

com.ibm.xsp.webapp.FacesServletEx.serviceView(FacesServletEx.java:193)

com.ibm.xsp.webapp.FacesServlet.service(FacesServlet.java:154)

com.ibm.xsp.webapp.FacesServletEx.service(FacesServletEx.java:137)

com.ibm.xsp.webapp.DesignerFacesServlet.service(DesignerFacesServlet.java:86)

com.ibm.designer.runtime.domino.adapter.ComponentModule.invokeServlet(ComponentModule.java:538)

com.ibm.domino.xsp.module.nsf.NSFComponentModule.invokeServlet(NSFComponentModule.java:1140)

com.ibm.designer.runtime.domino.adapter.ComponentModule$AdapterInvoker.invokeServlet(ComponentModule.java:803)

com.ibm.designer.runtime.domino.adapter.ComponentModule$ServletInvoker.doService(ComponentModule.java:758)

com.ibm.designer.runtime.domino.adapter.ComponentModule.doService(ComponentModule.java:527)

com.ibm.domino.xsp.module.nsf.NSFComponentModule.doService(NSFComponentModule.java:1124)

com.ibm.domino.xsp.module.nsf.NSFService.doServiceInternal(NSFService.java:513)

com.ibm.domino.xsp.module.nsf.NSFService.doService(NSFService.java:342)

com.ibm.designer.runtime.domino.adapter.LCDEnvironment.doService(LCDEnvironment.java:304)

com.ibm.designer.runtime.domino.adapter.LCDEnvironment.service(LCDEnvironment.java:261)

com.ibm.domino.xsp.bridge.http.engine.XspCmdManager.service(XspCmdManager.java:291)

java.lang.ClassCastException: java.util.TreeMap incompatible with java.util.List

javax.faces.component.SelectItemsIterator.next(SelectItemsIterator.java:172)

javax.faces.component.UISelectOne.matchValue(UISelectOne.java:167)

javax.faces.component.UISelectOne.validateValue(UISelectOne.java:140)

com.ibm.xsp.component.UISelectOneEx.validateValue(UISelectOneEx.java:364)

javax.faces.component.UIInput.validate(UIInput.java:688)

com.ibm.xsp.component.UISelectOneEx.validate(UISelectOneEx.java:343)

javax.faces.component.UIInput.executeValidate(UIInput.java:893)

javax.faces.component.UIInput.processValidators(UIInput.java:453)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

com.ibm.xsp.component.UIDataPanelBase.processValidators(UIDataPanelBase.java:371)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

com.ibm.xsp.component.UIDataPanelBase.processValidators(UIDataPanelBase.java:371)

javax.faces.component.UIForm.processValidators(UIForm.java:194)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1215)

javax.faces.component.UIViewRoot.processValidators(UIViewRoot.java:383)

com.ibm.xsp.component.UIViewRootEx._processValidators(UIViewRootEx.java:1432)

com.ibm.xsp.component.UIViewRootEx.processValidators(UIViewRootEx.java:1408)

com.sun.faces.lifecycle.ProcessValidationsPhase.execute(ProcessValidationsPhase.java:97)

com.sun.faces.lifecycle.LifecycleImpl.phase(LifecycleImpl.java:210)

com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:96)

com.ibm.xsp.controller.FacesControllerImpl.execute(FacesControllerImpl.java:250)

com.ibm.xsp.webapp.FacesServlet.serviceView(FacesServlet.java:206)

com.ibm.xsp.webapp.FacesServletEx.serviceView(FacesServletEx.java:193)

com.ibm.xsp.webapp.FacesServlet.service(FacesServlet.java:154)

com.ibm.xsp.webapp.FacesServletEx.service(FacesServletEx.java:137)

com.ibm.xsp.webapp.DesignerFacesServlet.service(DesignerFacesServlet.java:86)

com.ibm.designer.runtime.domino.adapter.ComponentModule.invokeServlet(ComponentModule.java:538)

com.ibm.domino.xsp.module.nsf.NSFComponentModule.invokeServlet(NSFComponentModule.java:1140)

com.ibm.designer.runtime.domino.adapter.ComponentModule$AdapterInvoker.invokeServlet(ComponentModule.java:803)

com.ibm.designer.runtime.domino.adapter.ComponentModule$ServletInvoker.doService(ComponentModule.java:758)

com.ibm.designer.runtime.domino.adapter.ComponentModule.doService(ComponentModule.java:527)

com.ibm.domino.xsp.module.nsf.NSFComponentModule.doService(NSFComponentModule.java:1124)

com.ibm.domino.xsp.module.nsf.NSFService.doServiceInternal(NSFService.java:513)

com.ibm.domino.xsp.module.nsf.NSFService.doService(NSFService.java:342)

com.ibm.designer.runtime.domino.adapter.LCDEnvironment.doService(LCDEnvironment.java:304)

com.ibm.designer.runtime.domino.adapter.LCDEnvironment.service(LCDEnvironment.java:261)

com.ibm.domino.xsp.bridge.http.engine.XspCmdManager.service(XspCmdManager.java:291)

We developed this application in notes 8.5.1 designer. Is there someone who encountered the same error and does have a solution?