Customizing statements uses an embedded browser window instead of a standard application window.
This appears to be an issue related to the embedded browser Intuit recently wrote and included in QuickBooks updates - to replace Internet Explorer at the last possible moment, even though MS announced its end of support date over a year ago.
That new browser seems to be very error prone and causes crashes as well as the cases where buttons don't do anything and/or expected windows don't appear when trying to use various features that use browser windows.
Until Intuit fixes this mess, there probably isn't any workaround.