We get this every day in our installation and suspect it might be linked to WHD coming to a complete halt daily.
After we get this exception in our install, within roughly 40 min WHD will stop responding to HTTP requests. We end up rebooting the server to expeditiously get WHD back online. This happens about twice a day. We have roughly 12,000 clients, and 40 tech/admins. The server runs reasonably fast before this happens, then it grinds to a halt and it is sudo reboot time. Our server runs on AWS and started this when we moved to AWS from Mac OS X (big mistake for other reasons).
Interestingly enough, I just reproduced the problem on the live online demo.
Anyone know how to prevent this? Some kind of workaround or even a fix for the problem?