Firewall blocking backup to NAS

Hello,can you please help me? I recently had to reinstall my firewall, and it now appears to be blocking my Acronis backups to NAS.

I have Acronis True Image Home 2012 running on a PC with Windows 7 Professional and BitDefender Internet Security 2011. I run daily scheduled Acronis backups of My Partitions and selected data to a networked QNAP NAS storage unit. This process was working until recently when I had to reinstall BitDefender. Now I get an error message, which says that Acronis is unable to find the backup version. I have tried creating a new backup, but the destination options do not include the NAS. However, both Explorer and my browser can find the NAS and I can log in, so it is only Acronis that can not find it.

Nothing is showing in BitDefender's firewall logs, but if I turn off the firewall the backup process is able to complete successfully. I have obviously forgotten something that I need to set up in BitDefender's firewall settings, but I can't see what it is. Can someone please suggest what I need to do?

Thank you.

Extract from Acronis Backup report:
The system cannot find the path specified (0xFFF0)
code = 80070003)
Tag = 0xBD28FDBD64EDB8BC.
41 Information 3/8/2012 11:10:37 AM Error occurred while opening the file.
42 Information 3/8/2012 11:10:37 AM Logon failure: unknown user name or bad password
43 Information 3/8/2012 11:11:07 AM Error occurred while opening the file. (0x40007)