I am running Backup Exec 10 Rev. 5484.
All backups from a remote server were failing with: Final error: 0xe00084af - The directory or file was not found, or could not be accessed.
After some research on Symantec's site and other forums I found that the SQL Server WSS Writer service on the remote server was causing the issue. With this service started backups fail, with it stopped backups are fine. I believe in my case this service was installed with SQL Server 2005 Express. Other forums have mentioned WSUS installed it. This service does not need to be running in order for manual backups of the databases to be performed within SQL Server Management Studio Express.
Apparently, upgrading Backup Exec to a newer version fixes this conflict.
For now, I disabled this service until we upgrade to a newer version of Backup Exec.