iMac 2019 and Denon AV receiver

I have problems with connecting my iMac (2019 Retina 4k 21.5po 4096-by-2304 resolution, MacOS Catalina 10.15.4) on my Denon AV Receiver (AVR) s750SH and Epson 1450 projector. To make it clear, input comes from the iMac to the AVR, then AVR outputs to the projector and then projector displays images. I have no problem with input sources other than my iMac (DVD player, PC, or cable/sat). I m not sure where the problem is coming from, but I suspect it may be between Denon AVR and iMac. Here is the description of my installation: My AVR s750SH is connected to an Epson 1450 projector. My iMac computer is connected to AVR by an HDMI wire (ultra HD 48 Gbps compatible) through an USB C to HDMI adapter (4K 30Hz). The adapter is plugged in the Thunderbolt 3 of the iMac (since there is no HDMI port on the iMac). The HDMI cable is plugged in input connector reffered as "media player" of the AVR. The problem is that when I select the "media player" channel of the AVR to display the image coming from my iMac, the image of my iMac screen appears but is "on and off", like if there was a signal problem. The 4k Signal Format setting in the AVR is set to "Enhanced" as recommended. I tried to plug the HDMI cable in other input connectors of the AVR s750SH, but it was the same problem. I also tried to unplug the HDMI cable of the AVR and re-plugged it on another monitor, in order to check if there could be something wrong with the HDMI cable and the USB C to HDMI Adapter, the result is that is absolutely no problem on this monitor. (NB : I have not tried to connect the HDMI from the iMac to the Epson 1450 projector, it is a bit tricky to do because the projector is mounted on the ceiling.) Would you have any recommendation to fix that? Thanks