This sounds like a problem I just had. No tuners were in use prior to the problem. I was switching between local channels delivered by two hd homerun tuners. I have a 2-tuner extend and 4-tuner quatro; both with identical channel listings.
Did a controlled test to see if I can reproduce the error. Rebooted Channels server, terminated the Channels app and rebooted Shield.
Switched between channels to reproduce the error. After surfing from one channel to another, I got the playback failed error. My hd homeruns show the channels in use. I exited the channels app on the shield but did not terminate it. Tuner sharing is on in the Channels app.
While the error active, on the settings tab for the Channels server, the Activity information shows the following (nothing should have be active):
Watching ch15.1 ABC15 from 192.168.1.150.
Watching ch10.1 KSAZ-DT from 192.168.1.150.
Watching ch12.1 KPNX-HD from 192.168.1.150.
Watching ch5.1 KPHO-HD from 192.168.1.150.
Watching ch61.1 KASW-HD from 192.168.1.150.
2019/11/21 10:57:40 [TNR] Opened connection to TVE-Spectrum for ch6030 CNN [transcode=none]
2019/11/21 10:57:46 [TNR] Opened connection to 1078E283/0 for ch15.1 ABC15 [transcode=none]
2019/11/21 10:57:47 [TNR] Closed connection to TVE-Spectrum for ch6030 CNN
2019/11/21 10:57:50 [TNR] Closed connection to 1078E283/0 for ch15.1 ABC15
2019/11/21 10:57:53 [TNR] Opened connection to 1078E283/0 for ch5.1 KPHO-HD [transcode=none]
2019/11/21 10:57:56 [TNR] Closed connection to 1078E283/0 for ch5.1 KPHO-HD
2019/11/21 10:57:59 [TNR] Opened connection to 1078E283/0 for ch61.1 KASW-HD [transcode=none]
2019/11/21 10:58:03 [TNR] Closed connection to 1078E283/0 for ch61.1 KASW-HD
2019/11/21 10:58:06 [TNR] Opened connection to 1078E283/0 for ch5.1 KPHO-HD [transcode=none]
2019/11/21 10:58:10 [TNR] Closed connection to 1078E283/0 for ch5.1 KPHO-HD
2019/11/21 10:58:12 [TNR] Opened connection to 1078E283/0 for ch15.1 ABC15 [transcode=none]
2019/11/21 10:58:18 [TNR] Closed connection to 1078E283/0 for ch15.1 ABC15
2019/11/21 10:58:21 [TNR] Opened connection to 1078E283/0 for ch5.1 KPHO-HD [transcode=none]
2019/11/21 10:58:24 [TNR] Closed connection to 1078E283/0 for ch5.1 KPHO-HD
2019/11/21 10:58:27 [TNR] Opened connection to 1078E283/0 for ch15.1 ABC15 [transcode=none]
2019/11/21 10:58:31 [TNR] Closed connection to 1078E283/0 for ch15.1 ABC15
2019/11/21 10:58:34 [TNR] Opened connection to 1078E283/0 for ch61.1 KASW-HD [transcode=none]
2019/11/21 10:58:37 [TNR] Closed connection to 1078E283/0 for ch61.1 KASW-HD
2019/11/21 10:58:39 [TNR] Opened connection to 1078E283/0 for ch10.1 KSAZ-DT [transcode=none]
2019/11/21 10:58:41 [TNR] Closed connection to 1078E283/0 for ch10.1 KSAZ-DT
2019/11/21 10:58:44 [TNR] Opened connection to 1078E283/0 for ch12.1 KPNX-HD [transcode=none]
2019/11/21 10:58:47 [TNR] Sharing existing connection to 1078E283/0 for ch12.1 KPNX-HD (clients=2, len=0, transcode=none)
2019/11/21 10:58:50 [TNR] Sharing existing connection to 1078E283/0 for ch12.1 KPNX-HD (clients=3, len=0, transcode=none)
2019/11/21 10:58:53 [TNR] Sharing existing connection to 1078E283/0 for ch12.1 KPNX-HD (clients=4, len=0, transcode=none)
2019/11/21 10:58:57 [TNR] Opened connection to 1078E283/1 for ch10.1 KSAZ-DT [transcode=none]
2019/11/21 10:59:00 [TNR] Opened connection to 1078E283/2 for ch61.1 KASW-HD [transcode=none]
2019/11/21 10:59:04 [TNR] Opened connection to 1078E283/3 for ch5.1 KPHO-HD [transcode=none]
2019/11/21 10:59:07 [ERR] Could not start stream for 1078E283 ch15.1 ABC15: HDHomeRun: 805 All Tuners In Use
2019/11/21 10:59:07 [TNR] Opened connection to 10589EA5/0 for ch15.1 ABC15 [transcode=none]
Going back to the shield (on the home screen) I double click home and terminated the channels app the Activity information now shows Idle and everything works. hd homerun tuners changed to idle as well.
This "feels" like a Channels App issue not a server issues, but I don't know that.
One last observation - I get more playback failed message on screen than show in the log.