Since I couldn't get rid of the error messages in the console no matter what I did in the CloudKit dashboard I decided to create a new CloudKit container and now the error messages are gone. So it definitely seems like some kind of server issue on CloudKit side.

I have the exact same issue.

Wow that's pretty incredible this bug snuck into iOS 18.

I would also like to know that.

Figured out I also had to had indexes for modifiedTimestamp to be sortable.

I figured out what was causing this error. I hade previously set com.apple.developer.icloud-container-environment to Production. When I deleted this it worked fine.

I have no answer unfortunately but would like to know if you solved this somehow?