I'm not sure how I fixed it, but it's fixed.
I shut down, unplugged everything, let it sit for a few minutes, plugged everything back in again (including the camera) and started it up. Then it worked.
I suspect the camera ribbon was not fully connected or something like that.
You can use an HDMI to Composite Video grabber together with an Composite Video to USB 2.0 grabber. The Pi is able to work with the signal from the CV to USB 2.0 grabber, if the chip is supported. If you search on your favourite webshop for "UTV007 USB Video Grabber", you'll find one.
I just wanted to say a big THANK YOU to Andy Johnson and his Single/Dual link DVI-D suggestion.
My PI4 would never connect to a Sony SDM monitor despite all config files changes I could find on forums. After ordering a new HDMI/DVI cable SINGLE LINK, it worked immediately like a charm.