Just to save someone else from this headache, I want to share the solution to a problem I encountered.
I tried using VEB to backup an Exchange 2013 server running on Server 2012R2.
It kept on failing with 'RPC service unavailable'. It turned out that installing Exchange 2013 (CAS role) changes the RPC port to 6005/tcp instead of the default 49152/tcp.
To solve this, add a DWORD registry key in HKLM\Software\Veeam\Veeam Endpoint Backup with the name EndPoint_Rpc_Transport_Port
Put the decimal value 6005 in and restart the Veeam Endpoint service.
Backups will now succeed.
This might be applicable to other environments / applications. To determine the correct RPC port use netsh interface ipv4 show dynamicportrange tcp
Perhaps Veeam should put this in a KB article?