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.
2014 remastered not opening in steam
in Rocksmith Remastered
Posted
Yes, the custom DLC songs appear but don't work