I never found anything useful in the logs, or any better way to reproduce it than wait between 2-7 days for devices to stop working. If I'd had better luck coming up with a cause / error message, I'd have logged an error, but felt embarrassed to. Not I'm sorry that I didn't share what I found to help others that it was affecting. I initially just rebooted the router when it flared up, but eventually got fed up and tried restarting daemons one at a time till things worked again. Hope this helps.