When using multiple displays, the expand and collapse (+ and -) is frozen on the Resolve Differences window and the window becomes unusable.
There is a known issue in erwin, where sometimes using an external display in addition to a laptop screen, can cause the resolve differences window to become dysfunctional on the external display, but still functional on the laptop display. This usually only occurs if the external display is set to be the main display in display settings.
The workaround to make this work on both displays is to set the laptop screen to be the main display and to set the external monitor to be the secondary. To do this, right click on your background and go to Display Settings. Then scroll down to the Multiple Displays section and make sure the laptop screen has "Make this my main display" selected, like in the below screenshot.
If it does not, then select that option and save changes. The Resolves Differences expand and collapse should now work on both displays. If the laptop was already set to be the main display and you are still facing the issue, then set the external display to be the primary display and laptop to secondary and it should then begin to work.