@hmerk - I have found a work around to this problem. Hopefully it helps the author of this plugin as this has been an issue for me for like 8+ months - but i just had a chance to sit down and look at it tonight. It appears to be a timing issue when other plugins are starting and the upnp framework not being completely available.
More info i posted here: WEMO Insight Switch not initalized after start openHAB2
My current workaround: https://github.com/DigitalBites/smarthome/tree/fix_wemo-startup-issues
Shamelessly borrowed the concept from the Sonos binding. 
Let me know if this helps. I want to improve the startup time and test it a bit more and I’ll create a pull request.