MSBuild exited with code 123

I'm using MSBuild to run SandCastle after a VSS build to create a help (.chm) file. It seems to run fine (the .chm file works) but it throws an error "%SystemRoot%\Microsoft.NET\Framework\v3.5\MSBuild.exe /p:Configuration=Release Chem4WordHelp.shfbproj" exited with code 123. I don't find any code 123 in the documentation on the MSBuild site, so I don't know it this is important or not. Anybody seen this before?

I'm also going to ping Sandcastle since it's not clear who is throwing the error.

Thanx.