What is the cause and solution for this?
Most likely the memory window or extended monitor window is in the hidden state while more than the allowable number of windows are open at the same time.
As shown in the figure below, you can display a list of all currently open windows by floating any menu bar (excluding the title bar) and right-clicking inside the box outlined in red. The items that are marked with a check are visible windows. Add a check next to the window name(memory or extended monitor) that problem occurs to put the memory window( or extended memory window) in the visible state. Finally, close the memory window( or Extended Monitor Window) that the problem had occurred while it is in the visible state.
Note: The Extended Monitor Window is supported on some products.