External process not working

I'm trying to use external processes to monitor a server, but I persistently get the error "The return value does not match the expected value". I've pared back the settings to the simplest possible but still can't get it to work:

Executable Name = "cscript.exe"
Directory = "C:\Windows\System32\"
Command Line = "test.vbs"
Startup = "C:\test\"

The monitor credentials are those used for all the other 200+ monitors without problems; this user has admin rights on the IPM server.

The vbs script is simply "WScript.Quit( 0 )". It runs fine from a command prompt.

Bizarrely if I open a command prompt on the IPM server, using "run as" and the monitor credentials, the monitor immediately succeeds (without changing anything). As soon as I close the command prompt, the monitor fails.

I wonder if there might be a local security policy setting that I need to make for the monitor credentials, but I can't find any reference to this. Anyone got any suggestions?