External screen turns off after macOS boots - Mac Mini When I plug in the screen and I turn on the Mac Mini, I see the Apple logo and the OS loading. After macOS boots the external screen goes black.
I had the same issue with EV3237 Monitor. Also, when I was lucky that the Mac Mini somehow started to send a signal, then the screen would go black randomly for a few seconds while I was working on it.

I tried the PRAM and SMC thing - no difference. I tried HDMI - no difference. On the other hand, I had no issue on a LG 4k 27" Monitor.

Finally I found a way to stop this: On the EV3237, I turned off the power saving feature. Now, my Monitor will only be off when I turn it off. On the other hand, I have a stable screen now. Somehow the signal from the Mac Mini seems to initiate a power saving on some monitors I guess.

So try to turn off power saving on your monitor.