Thank you.
Just to clarfiy:
I tried to drag 2 images into the viewport.
It changed to 1D.
I sat it back to 2D.
I didn’t even notice the blue marker.
When I restored the view (thanks, Ctrl + 1 was right), it worked again.
I have no clue what the difference between the view with the blue marker and without it does, but now when I click an image in the 1D view, it shows up in this view again.