Ok.
It seems like the problem accured already yesterday and had nothing to do with the header.php work on.
+ when i change the theme to another one, title and price appear again in the preview. (e.g. xstore parent)
=> so we thought its the child-themes fault.
but, when i overwrite the xstore child theme folder with an old xstore child theme folder (over 1 week ago), the problem stays the same.
=> so it seems like the problem is not directly linked to the themes.
How to find out whats the reason?
Best regards