Two windows parallel on Mac

Hi There,
I have been using the Mac version of your app for a while and enjoy it a lot. A few days ago I experience a bug: there are two Portfolio Performance open in the same time (like 2 “levels” of the app), overlapping each other. I can close the one that shows the data of my portfolio but then the window behind him stays on my screen with grey background. Can you please help?

The recommendation in this German-language thread is to use “Reset GUI” from the Help menu.