Thanks, your suggestion helped somewhat… Now if I start NoMachine from the dock on my MBA monitor, then I do see a client window on my Thunderbolt monitor (but the window is of the size of the MBA monitor), please note that there is no NoMachine menu in the Apple bar of the Thunderbolt monitor (it show another application e.g. iTunes), it only shows up in the Apple bar of my MBA.
On the other hand, if I start NoMachine from the Thunderbolt display, then I see the NoMachine menu from the Thunderbolt display Apple bar, but then I don’t see any client connection window… It’s almost as if the client window is positioned relative to the display where I invoke NoMachine from, and if that position does not fall into the second display, then I can’t see it…