Thank you for your help. Shift-N does work.
The problem seems to be the slowness of Dorico in galley mode. It obviously reacts too slow to catch the double clicks.
When entering galley mode, the reaction speed of every input becomes so slow that it is nearly impossible to work. When I entered the galley view and return to the page view, the speed stays slow and I also can’t use double click in page view. When I close the complete program and start again without entering the galley view, I can work again.
I think the speed (/memory usage?) especially in galley mode needs to be looked into.
I am doing an orchestral score with 30 instruments, but I think a program like Dorico should be able to handle this on a new machine.