I got this from the FAQ:

I've registered on the website, but I cannot login.
If you're told your username or password is incorrect, it may be that you registered with a username that is not compatible with the Powder Toy. To keep usernames readable, only alpha-numeric (as well as '-' and '_') characters are allowed in usernames, any other characters in your username are replaced with a hyphen ('-'). Try using your adjusted username to log in. Other common issues are: Your password may have characters that you cannot correctly enter into the Powder Toy, in which case, you should change your password on the website in your profile; You may not have typed your username or password correctly (they are case sensitive). Another rare but possible issue is that the username or password is too long to be entered in the game login screen.

That might help. Whatever that '-' in your username is, is probably the problem. (or maybe not, it looks like you have logged in before, since you already have saves)

@User_58758 (View Post)
Try changing your password on the website, to something with only a-z, A-Z, and 0-9 in it, or just try changing your password to see if your old password still works. It might have gotten changed somehow, or you just forgot it. If none of this works, or your password was changed, try asking Simon for help, he might be able to find/fix the problem.

@User_58758 (View Post)
Send him a pm (conversation). There is a button on his profile to do that. link