“Diese Datei kann leider nicht bearbeitet werden.”

As I maintain two different websites, I usually test changes and new software versions in a “localhost environment” (currently Windows 11, XAMPP 8.2.12, WordPress 6.5.5) before installing them on the external servers.

The bad news:
In the “localhost environment”, the following test conditions always lead to the Theme Editor error:
– Uploading Enfold version 6.0 via WordPress -> Design -> Themes -> Add new theme -> enfold.zip
– Copy the folder “enfold” (version 6.0) directly into the directory “../wp-content/themes”.
– Using Firefox
– Using Chrome
– All plugins disabled

The good news:
After installing Enfold version 6.0 on the external servers, the error does not occur,
even though the file “../wp-includes/functions.php” on the external servers also contains the code
// Normalising the path for Windows servers
$file = wp_normalize_path( $file );
Note: This code was introduced with the current WordPress version 6.5.5!

I did some further investigation and found that in “theme-editor.php” the absolute path of “$file” contains backslashes,
for example
“D:\xampp\htdocs\website-name/wp-content/themes/enfold-child/style.css”.
The backslashes cause the error reported by
“$file = wp_normalize_path( $file );”.
Replacing the backslashes by normal slashes in “theme-editor.php” like this
“$allowed_files[‘style.css’] = str_replace(‘\\’, ‘/’, $style_files[‘style.css’]);”
can avoid the error.
Manual patching is unsatisfactory, but I did not find any other solution to the problem.
Note:
Surprisingly, the plugin-file-editor does not have this problem, because only relative paths are used, not absolute ones.
May be the (WordPress-/)Theme developers could take a look at this and adopt that solution.