You DO NOT break Cecille's curse a second time. You need to go do something else. Talk to the demon queen, talk to Cecille in the garden in the morning. Something else to advance the story.
So, I did get this working but I had to load a save file from before I started working on Cecille for the second time. Where I was at, talking to Cecille in the morning did nothing.
I think what happened previously was an accidental sequence break. Right after satisfying Sera's curse, I went to intrude on Erin/Cecille's date. This caused the new date interruption scene without anyone having mentioned it. Afterwards, I don't remember my conversation with Sera mentioning Cecille sleeping in her room. In the post-sequence-break save, I tried to get this to trigger again, but after the date scene it just skips to afternoon with no conversation.
So, to fix it, I had to load a save file right after my last Sera scene and go through the game without triggering the date scene until it was explicitly brought up. Then things proceeded as (I assume) normal.