When you load the Case form in CSM application you will get an error in the bottom "Read-only true not set on field contact_time_zone: cross scope access denied"
The issue is caused because of OOB Client Script ("Set timezone read-only if not new record").
This Client Script was designed to make the contact_time_zone field read-only when creating a new case.
The error message you are seeing is displayed because you do not have the (contact_time_zone) field configured on the form.
Add the (contact_time_zone) field to the form via Form Layout.