I've been doing a bunch of research regarding shared calendars and exchange for the Mac and I haven't been able to find a resolution.  I've tried deleting my account and identity, but as soon as I add back in the shared calendar's I get the same permission's error.  I did some research and I didn't see a conversation about error 19955 and was wondering if someone knew about that.  We have 2 Mac's running on the exchange server and both are having this issue.  I did resolve it once and that required uninstalling Outlook, deleting my account and identity.  That didn't work this time.  The PC's seem to be fine.