Thanks for the response - however, for better or worse, it's a moot point now as i'm exchanging the TV for another one.
Since I couldn't find much specific info on my situation here or elsewhere, I called Best Buy - they sent a tech out and initially were going to replace the mainboard, but since I was still in the exchange period with a warranty they're opting to just swap the whole thing instead.
In the meantime, the problem hasn't happened again, but we've only used the TV 3 or 4 hours since then also. This issue hasn't happened on the other 2 TCL sets we've had for the last 5 years or so (including one elsewhere in the house that we use daily), so i'm leaning towards it being some sort of hardware issue as well.
Still curious what would make the Roku software generate this error with no USB plugged in (unless it was related to the TV's internal wireless that's apparently hooked up by a USB cable also), but at this point just fingers crossed that it doesn't happen again with the replacement.