I'm having the same issue with my Canon Selphy CP900, I am using Canon Selphy CP color ink paper set KP-108IN. I even tried opening multiple brand new ink cassettes, following the exact directions in the manual, but the problem persists.
I've recently had the same issue. I've had the same model Selphy, haven't had issues, and then suddenly the printer won't recognize the brand new cassettes as being new. I hope someone from Canon can help with this issue.
