Hey,
I've just found a solution for this problem:
Edit perefences of all selected files on the window below:
Select checkbox Run as administrator (I have polish version of windows, sorry) and click Apply
Then click Change DPI setings below and set like this:
Then ok and apply changes.
Now you can run MentalOmegaClient.exe, go to the options and set like below:
...and now works with any res you want, windowed or fullscreen on Windows 10.
I hope it'll help some of you.
cheers