I have a theme, lets call it Master. I manually created a child theme, lets call in Master-Child and added some files into it. All worked well until a notification came that I should update the Master because of a new version. The child theme has failed to work since. I get an error message "Master theme does not exist" although "Master" is activated. I tried to rename, capitalize and lowercase all characters. Thanks a lot.
The only string to use the master is
example screenshot with the Theme "Black Label" as Master, no Child theme used. The Screenshot was in Adminer inside the WP install.