Did some testing on my 15.5 where I can assign booking manager GUI (on that 15.2 above I could assign one in the GUI)
I still do not see the bookingmanager tag in the sample XML file, but when I read out a resource which has a booking manager, that is in the output file.
No problem changing that with XOG
The same thing with Calendar. Can change the calendar with XOG.
The simplest possible things are typo in the calendar name and inadequate rights.
Can the user with which you do the XOGing change the booking manage and calendar in the GUI?