Right click on the failing job and select View Logs, alternatively right click on the Mailbox Agent within the Client, select View and then Backup History. At the filter dialog, press OK. You will now have a list of jobs, right click on a failed job, and select View Logs.
Most of the contents of the log will be irrelevant, however keep an eye out for any instances of FAILED. Hopefully it will give you an idea of what is causing the issue.