ninjadodo

Sounds like it was only broken temporarily and not on purpose:

Oh damn, you're right. I hadn't had time to also test playing the CLDC, but as you say if you try to start any you just get stuck in an empty stage with no notes. What a shame.

I was having the same problem just now and tried a few different things. I got it working again by just deleting the D3DX9_42.dll file that you're supposed to install for CDLC. Starts without problems after that. I did not need to change versions or switch to a different Steam branch or anything like that. However, despite the appearance of shenanigans, the custom DLC songs I had installed still seem to work fine (still showing up in my song lists) and I was even able to add new songs still by copying them into the appropriate folder, so that suggests this is not an attempt to break modding.