I have this very thing happen to me...or I used to have it happen. What I found was, for me (as prophesied by Bill), it had to do with the manner/connection with which I was connecting to the GDB from the MXD. If I was connected with the admin account (sa) I didn't have that problem, but something in my personal connection permissions makes me hit that same wall. I have never spent the time to figure-out which permission it is exactly...I just have my publication MXD's that are set-up with connections through the sa account & all runs smoothly.
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