I have the same issue. In my case I have two screens, one of them I use from time to time to watch films (the shared one). Then, if I close opera and after that I stop using this screen (win + P --> Use only PC screen), then the next time I open Opera it is trying to open in a position that is not visible (the old position in the screen that right now I'm not using.
I can fix it by using some adivces from some fellows here (Win + D), thanks @jiafoongwee a lot.
It would be nice if this bug can be fixed (please just check if the position exists, otherwise use a default one).