Based on the current SCL for Symantec BUExec 12.5, Backup Exec for Windows 12.5 supports Windows 2008 R2 with the release of HOTFIX 331998.

Windows 2008 R2 Support

- Added Support for Windows 2008 R2 Operating systems (File System, System State, Shadow Copy Components Only).

Windows 2008 R2 Features NOT supported

- Installing a full version of Backup Exec 12.5 as a media server on a Windows 2008 R2 Server.
[...]

Based on www.symantec.com/business/products/sysreq.jsp you fully support Microsoft Windows Server 2008. There is NO distinction between Windows Server 2008 and Windows Server 2008 R2.

Obviously my installation of BUExec 12.5 [Build 15.5.2213] for Windows Servers failed.

- Failed to find the ASP.Net Version Registration with Microsoft Internet Information Services (IIS).
- [...] failed to parse the SQL script [...]\sysdbupg.sql. The error code is Access denied.
- [...] failed to parse the SQL script [...]\sysdbupg.sql. The error code is Access denied.
- [...] failed to parse the SQL script [...]\sysdbupg.sql. The error code is Access denied.
- [...] failed to parse the SQL script [...]\sysdbupg.sql. The error code is (5)  Access denied. Error number 29504.

[...] V-225-53: ERROR: Failed to install SQL Express BKUPEXEC instance with error 5. [...]
[...] summary.txt

I am not willing to use workarounds. So what timeframe do you expect for full Windows 2008 R2 support/compatibility?
How do I get the full software installation once it is available?

Yours sincerely
Andreas