When opening a project the VST-connections allways have to be loaded from a preset again to be able to work. They are allways kicked out when opening a project. Even when using the same preset in every porject and also when closing and immediately opening the same project again.
I would try changing my audio Hardware driver in the VST setup. Change it to a different device like generic ASIO then close Nuendo and reboot. Reopen Nuendo and select your desired audio Hardware driver again.
Not sure that will help but it’s worth a try.