WordPress Error Notice:fread():read of 8192 bytes failed with errno=21 Is a directory in

I'm blogging with WordPress, but suddenly I can't edit articles or post new ones, and I got the following error message.I would like to recover so that I can edit and post new articles.

Notice:fread():read of 8192 bytes failed with errno=21 Is a directory in/home/c7006248/public_html/ondine199918.com/wp-includes/functions.php online6574

The server uses ConoHa WING and the blog theme is Affinger6.
You can view the article itself.
https://ondine199918.com/

I looked at the directory and files in ConoHa WING's file manager as shown in the error address.

The error line 6574 is indicated by

$file_data=fread($fp,8*KB_IN_BYTES);

In , the peripheral code is

*/
function get_file_data($file,$default_headers,$context='){
    // We don't need to write to the file, so just open for reading.
    $fp = fopen($file, 'r');

    if($fp){
        // Pull only the first 8 KB of the file in.
      $file_data=fread($fp,8*KB_IN_BYTES);


        // PHP will close file handle, but we are good citizens.
        fclose($fp);
    } else{
        $file_data=';
    }

appears.

I would appreciate it if someone could help me.
Thank you for your cooperation.

Note:
The current version of WordPress is 5.9, and I don't know if this is before or after the error occurred.