Are you testing both the Chromecast and the Chromecast Ultra on the same HDMI port on the television?
Are there any other HDMI ports on the television that you can connect the Chromecast Ultra to and see if that behaves any differently?
Not sure if you have taken a look at the following link or not, but a few things to try.
I can't think of any further steps that you could try, because normally there shouldn't be any problem to connect a Chromecast Ultra to a TV. The support can help you better in this case I believe, because they might have further troubleshooting steps to try or they can also help you in case the CCU is faulty and needs to be replaced. Can also be the TV, no idea, it's just weird that it works with a normal CC, but not with the Ultra.
Sorry for the late reply. I had to buy a new tv because the HDMI on the old tv was not compatible with the ultra. The tv was almost 10 year old. But i can enjoy Stadia now! Greetz