passwords and usernames are both case-sensitive. Also, do you have any odd characters in your password? I think things like that are just replaced with a dash '-' in the passwords.
I'm pretty sure logging in from the game is working, if it wasn't then probably the whole server would be down. I still think it's your password, try replacing anything that isn't a letter or a number with a dash just in case, and making sure you type it in correctly exactly (it's case sensitive ...)
Well ok, I just tested logging in in-game, and yes, it does work. so it's something you are doing. Also this server with the forums is the same one that's used to login I think. Stuff like saves the game gets from static.powdertoy.co.uk just to make it faster.