I’ve found out the solution, with the help of Gemini. I’ve reuploaded a fresh copy of the theme in version 6.7 and changed in the file “admin/admin-core/admin-page-posts-meta-boxes.php” the line endings from MAC (CR) to Unix (LF), according to this guidance:
The “Single Line” Delusion (Line Ending Mismatch)
If the file was saved or extracted on x10hosting using non-standard line breaks (like legacy Mac \r instead of standard Unix \n), the PHP parser on your server won’t recognize where lines end.
To PHP, the entire file exists on Line 1. So when PHP encounters a syntax error anywhere deeper in the file, it gets blamed on line. If you are using an editor like Notepad++ or VS Code, open admin-page-posts-meta-boxes.php, set the line endings format to Unix (LF), and save. When you upload it back, PHP will give you the exact line number where the issue lives.