In this case, the customer got this error when he tried to render just a normal channel (with no Main channel):

' ERROR : Directory '.' is write-protected.
' ERROR : 21000-REND-RenderPasses - Unspecified failure - [line 258 in C:\Program Files\Autodesk\Softimage 2012 SP1\Addons\RenderManager\Application\Plugins\RenderManager.vbs]
' ERROR : 21000-CUST-RenderCurrentFrame - Unspecified failure

Now, ‘.’ usually means the current directory, and for XSI.exe the current directory is going to be %XSI_BINDIR% aka C:\Program Files\Autodesk\Softimage 2012 SP1\Application\bin. On Vista and Windows 7, it’s pretty common for users not to have write permissions in that folder. On my PC, I’m a member of the local Administrators account, which has write permissions, so I couldn’t repro the problem until I denied myself access.

Here’s a video walkthrough of using Process Monitor to troubleshoot this kind of problem: