I have this same experience, unable to click anything in the unit selection window when interface scaling is set to 125% and resolution set to 2560x1440. Reducing interface scaling to 100% allowed me to click things again, this is a problem as it prevents canceling the last built unit (especially if you are population capped) to get research going instead.
If I have a group of any units or buildings, and want to select or deselect any, then if I click on the top of the box the game doesn’t respond. It only responds if I click in the bottom left of the unit square.
@JexBall Yes with monitor at 2560x1440p and in-game UI HUD scaled to 125% I can’t select any units. Obviously this is a huge problem because I can’t cancel upgrades/units/etc or select units in the bottom middle UI grid.
So, I am forced to play on 100% UI HUD which is really small at this resolution.