Turns out we're lucky; the gearbox seems to be okay, just the so-called damper plate (between engine and gearbox) is broken. Fortunately this thing is much cheaper than a gearbox.
Jörg, the friendly mechanic could tell immediately what the problem was when he turned the shaft and i could take it apart myself.
So by the looks of it we're going to leave Carriacou on the 20th for Curacao where we pick up my very good friend Robin and where we also try to get our new damper plate.