Accepting calendar shares sometimes work. Mostly they don't for me. (They worked in M2).
Here is what happens. Someone wants to share their calendar with me. So they right click on their calendar, choose share, type my email address in, choose "view" mode, "standard email", then clock OK. I am sent a "share created" email. When I double click on the email to accept it, nothing happens. Well, actually something sort of happens. Instead of my email being shown with the "accept & decline" buttons, a little gray artifact (looks like a small bar) appears over the menus on the left. The artifact overlays the red "Zimbra" logo on the left (see attached image). I have configmed this both with Firefox 1.5 on OS X 10.4.4 & with IE on Windoze XP.
This didn't happen when running M2.