Gajim/Tkabber can neither grant owner nor admin privileges, yet JBother can. Having a look at at the code send, the reason becomes obvious.
For owner the following does not work:
http://www.jabber.org/jeps/jep-0045.html#grantadmin states that is should be http://jabber.org/protocol/muc#admin instead of #owner and the bare JID should be used.