How do I check the error messages in Retain?

I've received Retain Cumulative Error Report and it says Error messages: (These are not comprehensive. See the logs for more info.) 

com.gwava.archive.exceptions.RetainWorkerException: FATAL ERROR ! Exception on mailbox: xxxx Please contact the Administrator. Original error message: null , Code: d715at com.gwava.gweasysoap.EasySoap.login1( com.gwava.gweasysoap.EasySoap.( com.gwava.gweasysoap.GWSoapClient.( com.gwava.caapi.process.RetainDredger.processMailboxes( com.gwava.caapi.process.RetainDredger.processNormalJob(

How do I check the error messages in Retain?  What is the FATAL Error exception message mean?