Installed theme uses get_template_directory, breaks WordPress on Windows

Tom’s comment is consistent with my experience. But even if the mixed slashes are an issue, you’re not going to be able to fix it in any way other than by editing the parent theme.

This is because the string that contains the forward slash is not being passed through any WordPress function with a filter. If the theme used the more modern get_theme_file_path() function, then you would be able to fix the issue with this line of code in your child theme:

add_filter( 'theme_file_path', 'wp_normalize_path' );

The only other thing I would suggest is making absolutely sure that the file the theme is trying to load actually exists in that directory. An incomplete install or copy could have left you with missing files, which would cause this issue regardless of OS.