Solved with the 6.0.3 update: it was really related to the dual graphics card (Intel/Nvidia) on certain MacBook Pro models.
On a side note, my day yesterday was used at isolating two Nuendo bugs, this one and another one I haven’t mention yet: more communication from the Nuendo/Cubase team on the Nuendo forum would be appreciated, since, for example, the 6.0.3 version was frozen March 15th. I could have stopped chasing the GUI bug…