I had a happy installation of NextPVR, and then suddenly got a green screen on some channels.
Replaced the Hauppauge 1250 card with a 1609 quad. Same situation.

Then I remembered that I did a routine driver update to the video card... changed the hardware thinking that there had been no software changes.

Set the video renderer to Overlay (replacing EVR). Fixed the problem.
A video driver update on the XFX branded AMD Radeon R7 240 series, PCI-E x16, to 18.4.1 had been performed... that was the poison.

Not all bad. I now have a quad tuner in this system.

I have filed a problem report to AMD driver support, as follows:

-> Install the Hauppauge WinTV-1250 or WinTV-quad-1609 with Hauppauge driver package (8/24/2016, 1.58.34237.0) installed and hardware recognized.
-> Have both NextPVR 4.1.x and/or WinTV 8.0/8.5 installed on Windows 10 64-bit. Windows edition not critical.
-> These applications are known to be troublesome if the "Default" video profile is not selected, so be sure to keep "Default".
-> In the config of either/both of the PVR apps, the default Video Renderer type of "EVR" will fail, as follows:

You will have a blank green screen on some channels. Sound OK on all channels.

Setting the Video Renderer to "Overlay" restores the normal moving video. However, now OSD will not appear with the mouse cursor hovering in the video window for live TV. That's normal for the "Overlay" type, but it is a loss of functionality.

Radeon driver revs prior to 18.4.1 did NOT have this issue. This is a regression.

This was a problem for me, and it may affect others, hence this word to the wise.