So I can make, save and load levels with no problem when clicking the buttons in the top left. Clicking save prompts me to save, load prompts me to load etc. However clicking upload does nothing at all. I click it and nothing happens. The create button in the bottom right doesn't work either, in fact it's just a blank red box (and so is the ? box). Neither of those 2 boxes have any text in them, and I am unable to click them. Trying to click them just places a tile or a hazard (whatever panel i'm on) at that point on the screen.
How do I fix all this?
Chances are, you probably can't do anything about it.
It may be worth saving all your files from the SMB directory in Steam (including your game save and custom levels) as a backup and reinstall the game from scratch. That seems to me to be about the only thing you can do... even when it's working "as intended", things still won't work anyway, so it's kinda pointless imo.