com.documentum.fc.client.DfSysObject___PROXY cannot be cast to com.documentum.admin.object.IDfServerConfig

I am using webtop 6.7 sp1 , DA version is 6.7 when I access the da page I get this error :

An error has occurred.

com.documentum.fc.client.DfSysObject___PROXY cannot be cast to com.documentum.admin.object.IDfServerConfig

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Stack Trace:

java.lang.ClassCastException: com.documentum.fc.client.DfSysObject___PROXY cannot be cast to com.documentum.admin.object.IDfServerConfig
at com.documentum.webcomponent.admin.favorites.AdministrationFavorites.initializeDistributedContent(AdministrationFavorites.java:338)
at com.documentum.webcomponent.admin.favorites.AdministrationFavorites.onInit(AdministrationFavorites.java:114)
at com.documentum.web.form.FormProcessor.invokeMethod(FormProcessor.java:1615)
at com.documentum.web.form.FormProcessor.invokeMethod(FormProcessor.java:1500)
at com.documentum.web.form.FormProcessor.fireOnInitEvent(FormProcessor.java:1165)
at com.documentum.web.form.FormOperation.requestOnInitEvent(FormOperation.java:159)
at com.documentum.web.form.JumpOperation.execute(JumpOperation.java:113)
at com.documentum.web.form.FormProcessor.openForm(FormProcessor.java:221)
at com.documentum.web.form.WebformTag.doStartTag(WebformTag.java:127)
at jsp_servlet._webcomponent._admin._favorites.__administrationfavorites._jsp__tag0(__administrationfavorites.java:1589)
at jsp_servlet._webcomponent._admin._favorites.__administrationfavorites._jspService(__administrationfavorites.java:220)
at weblogic.servlet.jsp.JspBase.service(JspBase.java:34)
at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:227)
at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:125)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:300)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:183)
at weblogic.servlet.internal.RequestDispatcherImpl.invokeServlet(RequestDispatcherImpl.java:526)
at weblogic.servlet.internal.RequestDispatcherImpl.include(RequestDispatcherImpl.java:447)
at com.documentum.web.form.FormProcessor.dispatchURL(FormProcessor.java:2207)
at com.documentum.web.formext.component.URLDispatchBridge.dispatch(URLDispatchBridge.java:107)
at com.documentum.web.formext.component.ComponentDispatcher.mapRequestToComponent(ComponentDispatcher.java:458)
at com.documentum.web.formext.component.ComponentDispatcher.doPost(ComponentDispatcher.java:334)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at com.documentum.web.formext.component.ComponentDispatcher.doService(ComponentDispatcher.java:295)
at com.documentum.web.formext.component.ComponentDispatcher.serviceAsNonController(ComponentDispatcher.java:133)
at com.documentum.web.formext.component.ComponentDispatcher.service(ComponentDispatcher.java:114)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:227)
at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:125)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:300)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:183)
at weblogic.servlet.internal.RequestDispatcherImpl.invokeServlet(RequestDispatcherImpl.java:526)
at weblogic.servlet.internal.RequestDispatcherImpl.include(RequestDispatcherImpl.java:447)
at com.documentum.web.form.FormProcessor.dispatchURL(FormProcessor.java:2207)
at com.documentum.web.form.FormProcessor.doFreshInclude(FormProcessor.java:1084)
at com.documentum.web.form.FormProcessor.doFreshInclude(FormProcessor.java:1060)
at com.documentum.web.form.FormProcessor.redirect(FormProcessor.java:1012)
at com.documentum.web.form.FormProcessor.openForm(FormProcessor.java:259)
at com.documentum.web.form.WebformTag.doStartTag(WebformTag.java:127)
at jsp_servlet._webtop._classic.__emptycontentclassic._jsp__tag0(__emptycontentclassic.java:119)
at jsp_servlet._webtop._classic.__emptycontentclassic._jspService(__emptycontentclassic.java:88)
at weblogic.servlet.jsp.JspBase.service(JspBase.java:34)
at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:227)
at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:125)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:300)
at weblogic.servlet.internal.TailFilter.doFilter(TailFilter.java:26)
at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
at com.documentum.web.servlet.ResponseHeaderControlFilter.doFilter(ResponseHeaderControlFilter.java:319)
at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
at com.documentum.web.servlet.CompressionFilter.doFilter(CompressionFilter.java:84)
at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
at com.documentum.web.env.WDKController.processRequest(WDKController.java:98)
at com.documentum.web.env.WDKController.doFilter(WDKController.java:86)
at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:56)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.doIt(WebAppServletContext.java:3684)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3650)
at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321)
at weblogic.security.service.SecurityManager.runAs(SecurityManager.java:121)
at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:2268)
at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:2174)
at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1446)
at weblogic.work.ExecuteThread.execute(ExecuteThread.java:201)
at weblogic.work.ExecuteThread.run(ExecuteThread.java:173)