@bbubulka said:
Also, ensure WIN10 isn’t using the device itself. Checkout this thread: https://forums.plex.tv/discussion/274173/xb1-tuner-wintv-hvr-955q-cant-find-channels#latest .
Checkout @indyhill post on June 7 (PAGE 2) 
Thanks. Sorry I didn’t find that thread on my own; guess my forum search skills aren’t quite up to snuff (I did search). I checked Windows 10’s recording devices, and sure enough the Hauppage was set as the default. I plugged in an Xbox headset with a 3.5mm jack and set it as default, tried to scan again, but still no channels.
I went back to the recording device list and disabled it, having noted that Windows was still showing audio levels on the device even though it wasn’t set as default, but the scan failed again. Figure this may have been because disabling it prevents Plex from using it. Re-enabled with it not as default, and Windows now just shows it as “Ready” without actively showing the fluctuating levels that indicate it’s currently watching the incoming signal.
For good measure, I’ve also closed every non-essential background service, so all I have running now is PMS, qBittorrent, Sickbeard, SabNZBD, Chrome, and Radeon settings module (NOT any of the extra capture software, just the driver and basic control panel).
So that worked! I’m a bit concerned that restarting the server will bring the issue back, depending upon whether one of my usual background apps will try to grab it again. I got over-eager and started a recording of something right away, so can’t restart just now. But now that I know it was either one of those applications or the disable/re-enable trick in recording devices, I’m sure I can use process of elimination to find the culprit and determine a solution if the problem returns. To hedge my bets, I’ve gone ahead and installed WinPatrol to delay startup of the most likely culprits, in my case EventGhost, Steam, Belvedere, and Dropbox. I actually don’t think any of my background apps are probable as the cause; I think it was the Windows thing, but doing this will only help me in having the system boot faster and PMS to load more quickly.
So again, thank you so much for your help in getting this issue resolved. If I find anything new that may help others, I’ll come back here with an update.