I finally found it. It was my printer. I have a *lot* of network devices and this was not my first guess by a long shot.
Epson XP-7100
The solution was to log into the web interface as administrator, then:
Advanced Settings > Services > MS Network
Uncheck 'Use Microsoft network sharing'
The printer has the ability to share files on a plugged in SD card. I've never used this feature. Perhaps it gives a response Heresphere doesn't like when there's nothing plugged into the printer to share.
Hope this helps someone else in the future.