If you are uncertain as to the source, might want to start by launching a command prompt and issuing a ping -a XXX.XXX.XXX to identify the source. Once you have identifed the source, be it a front-end application, SSRS, etc. verify the credentials being used to authenticate to the database engine. Based on the last entry, you might want to take a look at the reporting services configuration manager and verify the credentials it is using to authenticate to the ReportServer database.

http://technet.microsoft.com/en-us/library/ms156305.aspx