Contacts window pops up "by itself"

Since nearly a month the following contact window pops up approx. every hour or so: The window doesn't seem to belong to a specific application, but when it pops up it steals the focus and keyboard input is eaten by it until I either click into the previous app or close the contact window with ESC. The window is free floating (despite the pointy bit on the left!) and it can float above any app's windows. Here an example where it floats over my Vivaldi browser window: I have no idea whatsoever where this pop up comes from. Does anybody know more? I'm running macOS 14.5 on a MacMini M2 Many thanks for any pointers!