My guess would be the power supply is putting out something closer to a square wave than a sine wave and the monitor power supply is unable to turn that into DC.

They market it as a pure sine power supply: https://eu.ecoflow.com/products/river-portable-power-station...

Is there another appliance that I can test to see if that's the issue? I don't have an oscilloscope.

Edit: I just tested powering it from a UPS: It's flickering more now and when it show the correct content it stays for a second more then when powering from the ecoflow.

The monitor model is: BenQ GW2780-T.

I would think the same for some products but AFAIK those Ecoflow batteries are quite fancy.

I printed a document using one and generally printers hate running off generator power.

Laser printers don't like them. It's because the fuser is high amperage. Nothing to do with power quality.

It sounds very likely that this might be electromagnetic interference. The good part is that it’s easy to troubleshoot, just move the generator (or the monitor, whichever is lighter) far away, like in another room or outside.

You can also try a different, thicker cable, if you have one handy. The thicker cables are usually better shielded by having a sheath of tiny wires to catch that interference and conduct it to ground.

I just tested moving the ecoflow to another room (both door closed ~5m between the ecloflow and the monitor) and used an extension cable to power the monitor.

The issue is still happening.

Unfortunately I don't have a different HDMI cable at hand now. I'll test another cable as soon as I get my hands on one.

I tested another HDMI cable and it's been running smoothly so far.