User Admin tab, error: You have requested an application that is stopped

Hi All,

My Portal is working Fine except this User Administration Tab,

When i Click on User Admin tab it Shows:

Error on Page:

503 Service Unavailable

Application stopped.

You have requested an application that is currently stopped

and in log file it gives:

#1.5#C0000A0F10F1006400000075000017A400044E1A86C2AB1C#1211775743919#com.sap.portal.portal#sap.com/irj#com.sap.portal.portal#J2EE_GUEST#2##sinnodsap01_E12_34388750#Guest#56e18e902adb11dd8054fca00a0f10f1#SAPEngine_Application_Thread[impl:3]_32##0#0#Error#1#/System/Server#Java###An error occured during authscheme computation.

[EXCEPTION]

#1#com.sapportals.portal.prt.runtime.PortalRuntimeException: Access is denied: com.sap.portal.runtime.logon.certlogon - user: Communication User,

AllServices are running fine in Visual Admin,

My UME is on Abap...

Urgently required some help in finding solution.....