It seems like GameJolt changed a little how it handles API calls as they don't allow MD5 hash being generated with an extra / character at the end. As it was reporting that the "Signature was invalid"
Example: It doesn't accept http://gamejolt.com/api/game/v1/scores/tables/?game_id=xxx&format=jsonyyyyyyyyyy it instead expect http://gamejolt.com/api/game/v1/scores/tables?game_id=xxx&format=jsonyyyyyyyyyy (notice lack of / character after the tables).
So basically a quick fix to the problem is to change Constants.cs and remove the / character in every API_XXX string except the API_ROOT so the call will not end with /.
Issue raised by @DerpVulpes on GameJolt