Definitely a Mint 21.1 issue.
I upgraded to 21.1 when live viewing TV would eventually stop, become unresponsive (I only use Theater View, btw) and crash Cinnamon after about 30 minutes or so of viewing. I was hoping the upgrade would solve the issue.
Now I get the "white screen" when I launch Theater View. I can watch TV in the standard view when the renderer is set to JRVR, but changing it to Legacy OpenGL results in a white screen even in standard view. BTW, you have to close MC and restart it after switching for the changes to take effect (in case anyone else wants to experiment). I tried installing the XFCE environment but it was functionally unusable and apparently there's something about having to install XFCE first, then Cinnamon, but I'm not sure and really didn't want to pursue that route anyway.
Anyway, just wanted to throw in my experience. So, the net-net is TV works in standard view, white screen in theater view. I'll be more than happy to post stats/deets/logs if it will be helpful.