During flashing you should not apply main voltage. But during boot you should. I had a similar problem. And is was resolved by using 230v.
If you swap L and N because you are using a power plug to test the device will boot.
But be careful trying to connect s1and s2.
In my case that resulted in power outage and a black device.
Hey @bkbartk it worked! Thank you very much! I have one fully functional device now. The problem was really the insufficient power supplied to the ESP board. When connected to main power it started without issue.