I've upgraded yesterday from 5.0.14 to 6.0.1OSS on Debian4.
Now I cannot add/edit appointments anywhere than the first calendar.
In the edit dialog, if I click on the cal chooser I get the list, but clicking "OK" has no effect.
Dragging an item to another cal produces an exception in mailbox.log.
Moving elements using Zimbra Desktop works...
I filed a bug for this: Bug 41732 – calendar chooser not working, cannot choose different calendar for appointment