When NUnit stops working on a build machine…
… then it’s time to check the registry entries. Make sure you see something like this:
In case the build runs in the context of a dedicated user, not the one who logs on to a build machine and install applications, then it is likely that the build user does not have correct setup for NUnit assemblies. You can of course import missing registry entries into that user registry part, but the best is just to create them for everyone under LOCAL_MACHINE folder using path \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727\AssemblyFolderEx.