I have the same issue and have been triaging around a few lines of enquiry. This may possibly help.
First, I am in the UK and have a VBox 3442 tuner and Plex (with lifetime pass) running on a Synology NAS. For the past year or so, I have been unable to watch - via Plex Live TV - a handful of broadcast channels with the familiar “Playback Error. Could not tune channel. Please check your tuner or antenna.” message.
Yet, I can watch all broadcast channels via the VBox TV app. So, I conclude, that the hardware is working well but there is an interface issue between the VBox and Plex. In the dim and distant past, I’m sure I was able to watch all these channels via Plex but when and what changed, I cannot be certain.
Second, I found a related thread at VBOX DVR tuning issue for Sydney Australia. This seems to suggest that the VBox is somehow passing incorrect information to the Plex server on how to tune to certain channels. However, given the variety of hardware tuners that have reported the same issues, could it be that the Plex server is where the address mangling is occurring?
I’m not really sure where to go next with this. The discussion I reference above suggests there’s a way to see what URLs are being used to access a live channel stream but I don’t know how to do this. Working out what URL Plex is using to access live streams for other hardware tuners might narrow it down as a Plex issue.
