macOS calendar | add google calendar - domain DAAccountValidationDomain / error 100

Hello people from the Apple Support Community,

I get this strange error message when I try to add a Google Calendar to my macOS calendar:

It works if I create a hotspot on my mobile device and try it there.

But I need to get it working in our company.

We run a firewall which is kind of restrictive and IT asked me what they should do to fix the problem.

The would allow traffic for our developers, but they don't see any blocked traffic.

I would like to know specificially what exactly we do have to allow in our Fortinet Firewall to let the calendar sync correctly.

Can anybody help me with that question?

I tried to google the problem but in the internet there are a few questions about this bug, but noone really solved it, thats the reason I ask here for support.

Like I've said, on hotspot without firewall no problem, but in our company the access is restricted and I would like to know what exactly we have to unblock to allow traffic. Is it Apple specific, or is it a google related problem?

Kind regards,

Daniel O.

[Edited by Moderator]