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Not sure if this is the only way to trigger it, but I was working on a song's concept (manager) and decided that I wanted to plan a date with one of my idols. Dragged her in, did the date, and then suddenly the song couldn't be worked on. I didn't cancel the concept process, and (if my memory is right) I couldn't drag the song at all.

Same thing happened to me.  closing and reopening the game does not help