Hi Laszlow,
I believe it is reserved (even if the green dot does not appear so - screenshots 1. & 2. attached). I have restarted ArchiCAD, and my computer, and just tested again with no luck.
I have both projects open and with everything reserved, I am also the only person online at the time and the reservation dialogue shows no conflicts.
There was an interesting thing, however. If I select a master in the right pane, and then select a master in the left pane, the 'import' button becomes available, and when clicked, a dialogue says not much and hangs for a while before disappearing (screenshot also attached 3. & 4.).
I just can't tell if I'm not doing something right or if it is a GS issue.