a COM.BUILDFORGE.SERVICES.COM api EXCEPTION OCCURED

I have installed build forge and it was working properly till today.
But when I try to add new user or do any task.it comes back to Login page.
So restarted build forge server..and I found the following error in catalina.out.log file


Jun 9, 2011 7:22:19 AM com.buildforge.services.server.Main idle
INFO: Blocking until the dispatcher shuts down...
Jun 9, 2011 7:22:20 AM com.buildforge.services.server.manager.SessionManager getAuthContext
WARNING: Login failed - token is for an external session
Jun 9, 2011 7:22:20 AM com.buildforge.services.server.api.APIServerConnection process
WARNING: !!!
com.buildforge.services.common.api.APIException: API: Authentication Error.
at com.buildforge.services.server.api.APIServerConnection.setAuthToken(APIServerConnection.java:462)
at com.buildforge.services.server.api.commands.AuthCommands.doLogin(AuthCommands.java:301)
at com.buildforge.services.server.api.commands.AuthCommands.invoke(AuthCommands.java:90)
at com.buildforge.services.server.api.APICommandProcessor.process(APICommandProcessor.java:275)
at com.buildforge.services.server.api.APIServerConnection.handleRequest(APIServerConnection.java:236)
at com.buildforge.services.server.api.APIServerConnection.process(APIServerConnection.java:150)



Please respond what could have gone bad and how to resolve this issue.