You will need to check the mcscript.log for the reason for the failure. That is in the c:\programdata\mcafee\agent\logs directory. Can you post the entire contents of that log after an update failure if you don't see the cause in it? You might want to blank out any IP's or system names or any other sensitive information. Otherwise you can open an SR with McAfee support for further assistance.
I ended up re-checking the System Temp environmental variables. I saw that TEMP and TMP were pointed to a nonstandard folder. I created the folder that McAfee was looking for in the folder that TEMP and TMP were pointed to and the error went away.