If someone encounters the same problem....
> Windows 10 PC
> NVIDIA GeForce
> an attached LG flatscreen
> using HDMI
> "Automatic Standby" of the Monitor is "Off", Windows 10 "Power & Sleep" settings are "Screen: When plugged in, turn off after Never" and Windows 10 "Screen Saver Settings" are "BOINC + Blank Screensaver After Never"
By default, BOINC runs normally, with the screensaver showing climateprediction progress bars and the BOINC logo.
This is fine as long as I keep the LG monitor powered on.
Once I power the LG monitor off, there is no way to get the LG back from "power save" mode. After pushing the power button and moving the mouse, the screen blinks once and then goes back to power save immediately, switching off.
This does not occur with a nearby Samsung. Indeed, exchanging the LG with a Samsung shows there is a nice full away PC on the HDMI cable, but the LG won't act on it.
The only way of this is a full power cycle of the PC. A reboot doesn't help.
I haven't been able to do anything about this except keeping the LG powered on.
It's probably not BOINC specific, but I didn't encounter this so far.