Hi eFe,
To backup a 64-bit Exchange Server from a 32-bit media server, the 32-bit Exchange Management Tools are required on the 32-bit media server.
As Microsoft does not support/provide 32-bit Exchange Management Tools for Exchange Server 2010, a 32-bit media server cannot backup an Exchange 2010 server.
Resolution -
Install Backup Exec 2010 on a 64-bit machine in order to backup Exchange 2010.
Note:
To protect Exchange 2010, Backup Exec must be installed on a Microsoft Windows 2008 SP2 64-bit media server or a Microsoft Windows 2008 R2 64-bit media server.
Refer to below mentioned link for more details on this issue...
http://seer.entsupport.symantec.com/docs/344569.htm
Hope this helps to resolve this issue...