calendar sharing exchange zimbra

[vmt]

Hello all,

I have zimbra and an exchange domain where I want to use calendar sharing. I installed webdav on exchange with no auth in the moment. I send then from outlook the ical/ webdav share. In zimbra I add a new calendar under local folder-> calendar and getting: The following URL is not currently reachable: webcal:/...ics
Content could not be retrieved. Make sure the following URL is an RSS/ATOM feed:

adding the remote calender under user, add calender then an error report is generated:
Error summary: failed to sync rss url

Also changing webcals to https give same errors.
From browser the .ics is accessible.

what Im doing wrong?
Can someone help?

tia
stefan