if you see this in the polling log it is most likley on a R mail bag. I have seen it twice once last week and today at my onsite. It is becasue there is no XZ out folder at the main. you have to run the XZ out at the main to created the XZ folder then it will work.
another Fix for this was that the main had a corrupted Z out folder. I renamed the Z out at the main and that worked fine.