We're running into the same user. I made sure the following:

- running SQL 2008 SQL Agent as a Windows admin account called "SqlService"

- ensured that that account has full access to the UNC directory

- I can create a backup file to a local folder through the job

- it warns me on the manual backup file that it can't verify the UNC location

- I tried mapping the UNC to a drive letter without success (cannot find the path specified)

- I tried logging in as myself (a windows admin, sql server sysadmin) to backup to any network drive (cannot find the path specified)

Just don't know how to proceed from here. I can't get a directory in the job, as asked by Steve Jones.