Hi,
I tested it again in the browser again.
And the current calendar isn’t working but any new created calendar in iCloud is working.
So I called Apple support and after almost an hour I got the answer that depite the functionality/options for public sharing like for any new/other (created) calendar in iCloud, the first 2 standard apple/icloud calendars (“home” & “work”) are not available to be shared by a public link but only by personal invite (by email). 
(Maybe you could but a warning or a disclaimer in your app or forum app description. So “only new/non standard/default created/provided by apple iCloud calendars are working through the public link. “
Thanks 
Red Iphonitis rigorosa : level 11
