Thank you so much for this — you found a real bug, and a nasty one.
It's a menu layout error. The class grid is 3 columns wide but the code was calculating its height as if it were 2 columns, so the START button gets drawn about 50px lower than it should be. In portrait that's just an ugly gap. In landscape it pushes the button completely off the bottom of the screen — which is exactly what you're seeing. The map and class cards sit higher up, so those still work, but START is literally rendered outside the visible area. That's also why the screen felt "bigger than it should be."
Already fixed on my end. Coming in the next update.
In the meantime: turn your phone to portrait and it should start fine.
Really appreciate you taking the time to describe it that precisely — the "I can select options but can't start" detail is what let me find it.