Windows上的服务iCal文件到Outlook> 2016的问题

Question

I'm currently having an Issue with my Windows Outlook Client (within a VM). I'm just testing and Developing an Solution to Dump Event Items from my PHP Based APP to Outlook for the Project Admins. (Like scheduled Downtime etc.)

My Admins mostly use Windows based Machines. They are using Outlook 2016 and 2019. Both of them can't register for the iCal Calendar as it returns with an 401 Response. After researching it leads to an issue with the Microsoft Modern Authentication. As far as I understand it, the Server (Apache 2.4) does not know what to do with the request, so it returns 401.

This is the logic that outputs the ical file to the browser:

header('Content-Type: text/calendar; charset=utf-8 ');
header('Content-Disposition: attachment; filename="calendar.ics"');
print $calendar->render();

The renders the calendar into the required format. (Also Supports xml for external services)

I searched through a lot of pages, but none gave me a clue who I can buypass this issue.

The Solution that Microsoft Purpouses isn't an valid Solution for me, as some of my admins only have an imap account that does not support the features of outlook.

Any help is really appreciated.