Why might wsusutil export to a .xml.gz file produce a 0 byte output?

We're doing monthly updates to a disconnected network with Server 2012 installed on all servers.

Last month (February), the export created a 60 megabyte .xml.gz. Today, it creates a 0 byte file timestamped at the start of the export and the log's timestamp hits at the end of the process.

Any ideas where to look next? Google just returns results for the .cab 0 byte issue.