The phpCalendar administration screen now displays "File Security Status" for calendar.php, config.inc.php and license.php. After clicking on the help button, it's suggested that I set the file permissions to 644 ... They are already 644 and I'm getting a red Not Secure.
I only noticed this today after upgrading to 6.3.18.
I'm having the same problem. I've even changed my files to "444", and it still shows "Not secure" in the Setup Manager. I have viewed this in two different web browsers and both say "Not Secure" for those three files.