I am experiencing the exact same issue for a while now. I have been trying both solutions suggested in this thread, none of which worked for me.
with the last solution suggested, i had some difficulty trying because i have 2 screens with a very different aspect ratio, but when i finally got it working, still either openlp freezes as soon as i open a powerpoint, or if i unchecked the box for letting powerpoint control size and display for presentation, it just gives me 1/4 of the screen.
I noticed that if i open the openlp screen settings, it at all times displays a resolution that is totally different from what im actually running and what i see when i go to windows display settings.
When im running my normal resolutions (3840X2160) and (3840X2400) open lp tells me its (1920x1080) and (1280x800)
In which i find it interesting that for the first screen, the resolution that openlp says it is, is exactly half of the actual resolution, but for the second screen, its totally different.
When i changed both of my screens to the exact same resolution: (2560X2048) openlp thought that the one screen was indeed (2560X2048) but somehow decided the other one was (853X683)
Openlp every now and again prompts me with the mismatch in display settings message at startup as well.
Might this have anything to do with it?