Floating platform isn't on high enough in the Lumos challenge, gap can't be crossed

It seems you're not the only one to encounter this problem. According to this GameFAQs thread, you can bypass this by using a cheat:

Type in harrydebugmodeon and do a super jump. You'll be able to find the codes on the cheats section for this game.

So you type harrydebugmodeon to enable cheats. Then you type harrysuperjump to increase jump height. Once you've reached the platform, you can type harrynormaljump to set your jump height back to normal. You would then hit F7 to disable cheat/debug mode.

I also see people talk about running the game in various compatibility modes, but that doesn't seem to solve this particular problem reliably.

For other people encountering this: Don't break the 2 vases that you see at the bottom of the 3rd platform. Breaking these causes the platform to be stuck at a lower position. I think this only occurs on modern PC's because the pieces broken from the vases are still floating in mid-air, on old PCs with windows98 or windowsXP they should disappear. I've played through the entire game and that seems to be the only place where those floating pieces are causing issues.