I have an LG TV made about 2010. I've never had problems playing downloaded TV programs on USB. But I've had problems with this Ring Cycle set which has been released on iPlayer. It may be it's in higher definition than the stuff I've played before, and/or it might be due to a change in the BBC's downloads.

I found that a whole programme - they're all 3+ hours long - is always reported as 'invalid' by the TV. A short extract - the first 5 minutes say - will play.

After a lot of trial and error I've established that if I keep the length to 1 hour, and also downscale the definition, I can produce a reliably playable file. I use a command like:

ffmpeg -i infile.mp4 -vf scale=-1:720 -ss 1:00:00 -to 2:00:00 outfile.mp4

I now have 13 one-hour parts to watch over the next several months!

LG TVs are apparently notorious for this. Maybe other brands too.