Does this still happen to y'all if the preference "Show reminders for past-due meetings" is unchecked?
I can easily reproduce the reminders-for-shares problem with that setting checked, but not if it's unchecked. Startup is certainly less annoying without past-due reminders.
To set at command line, zmprov ma
user@example.com zimbraPrefCalendarShowPastDueReminders FALSE and then shift-reload the browser session.