I have a Surface RT and it works with my 626. I've been successful with RT 8.0 and 8.1 versions.
I've found that it works best to turn on the receiver, press the Bluetooth button and then start the Surface. This almost always connects. This assumes you've paired already. I've had the driver error issue but it will go away with a restart.
There is a Bluetooth setting on the Surface for power that you should remove... keeps the Bluetooth from turning off on low power/battery.
I think the issue is part Onkyo and part Surface. My Windows phone pairs great. I also pair with my Surface Bluetooth headphones from both Klipsch and Jaybird and they never drop.
I liken the Onkyo issue with the way the receiver doesn't gracefully handle changes between TV/cablebox audio changes such as going from a show to a commercial. It would be great if Onkyo could do a bit more testing.
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