I think I found the culprit.
My Windows 7 VM lost it's network connection. Somehow, Clay.io has it's issues with being used offline (makes sense, since some of the scripts it accesses are placed on their servers). I fixed this by adding a login button to the main menu and adding "Is Online" browser conditions to all clay.io actions, so they only fire when the machine is really online.
PS: Has anyone tried the Desura API? I hear it's mainly XML and Ajax, so I guess that would work for HTML5 games too?