retension should not have any effect ...but still you can remove the retention and try runnign the backup again ..also try using a different tape and check if any error reported int eh event log
Go to your Device Manager and check the driver for your tape device. It should be a Symantec driver. If it is not, then in your BE console, go to Tools --> Wizards --> Device Configuration Wizard to install the Symantec driver for your tape device.
At this time Backup Exec 12.5 is not supported with Windows 2008 R2 as a Media server. If you would like to backup a Windows 2008 R2 server there has been a hotfix release that added remote agent support for Windows 2008 R2. Windows 2008 R2 will be supported with Backup Exec 2010 when it is released. I have provided a link below that discuss how to participate in the Beta for Backup Exec 2010.