Well,
As we have make as many tests as a man can think off, we are pretty surry that this is a bug of 5.0.GA version
It seems that the sub address books are not shared at all, as the client side of the shared address book have a add sharing button on the used shared address book, that should not normally have, and if he tries to share it again by itself (share the shared address book.. nice one

) the following error pops up
Could not send message due to invalid address(es) msg: Invalid address: undefined
code: mail.SEND_ABORTED_ADDRESS_FAILURE
method: ZmCsfeCommand.prototype.invoke
detail: soap:Sender
data: [object Object]
Any help?