I only noticed as I had come in from outside and wanted to check a couple of devices, but the hub GUI was down. The hub light was green. It was not connected to the router.
And it appears it wasn't doing ANYTHING from 8:49 AM until I rebooted at 12:36 PM
I saw that same error recently after installing/uninstalling a 2nd instance of my Hue Hub. I can only presume they're related, mind you. I used to see that frequently when I used to have Sonos One speakers in the mix, too. FWIW