I've looked there is problem with Ajax interface in Opera v.10.53. Zimbra is open source v. 6.0.6. I've check that in two computers and on both is the same problem. First.. when You want to log in, there is a message "Please note that your web browser or display does not fully support the advanced version. We strongly recommend that you use the standard customer." Resolution on both computers is 1280x1024. When I force choose an Ajax interface after login I've see an error message "An error occurred while initializing zimletu „com_zimbra_email”. Notify the administrator." And then interface works strange slow and the address book doesn't work at all. When I type address book tab I get error "Undefined variable: ZmAddrBook."
Send this message may be to developers because I think this is important.