Mac Mini M1 disconnects from second screen

I have a setup with two projectors connected to a Mac Mini M1. One is connected to the HDMI port, the other is connected to one of the USB-C ports, with an USB-C to HDMI adapter. The projector disconnected to the USB-C port disconnects after a while. Generally it happens after 1-3 hours. Both projectors are running full screen applications, that I wrote myself. It only happens on the USB-C port, and it happens the same way on three similar setups, with each a Mac Mini M1 and two projectors.

I have tried changing the adapters for another brand, which didn't work. I'm using the software Amphetamine to keep the Macs awake, without allowing any display sleep, or other closedown procedures. This has always worked fine for me before, without any issues.

Any ideas what this could be?