Possible reasons for no video from AVR to TV?

[kinosfronimos]

Hi all, am trying to troubleshoot the reason for no video being sent from my AVR to my TV.
Setup was working perfectly yesterday, and every day before that.
Everything is left unplugged when not in use, and no surges nor thunderstorms since yesterday.

Setup is as follows:
Source: Android TV box (HDMI out) -> (HDMI in) Denon x3400h (HDMI out) -> (HDMI in) Samsung TV

Have tested the HDMI cables (they work).
Have tested sound from TV Box -> AVR -> Speakers (this works).
Have tested TV box as video source connected directly to TV (this works).

So sound is sucessfully processed from TV Box by AVR and output to speakers.
I do not want to run sound from Source -> TV -> AVR, because the TV is old, and sound would have to be over optical.
I'd like to avoid a factory reset if possible, as I don't currently have the time to set up audessy etc.
I have not hot-swapped HDMI cables (complete power unplug every time HDMI cables are swapped).

I've tested all 3 HDMI outputs on the AVR, with no video being sent to TV.

Any ideas what's up with this?
Possible solutions?

Thanks!