ALT+CMD+SPACE Suddenly works differently
As far as I recall ⌘n has always been the Finder shortcut for New Window assuming that Finder is the active app, ⌘⇧n opens the Finder with search focused globally. See the 'Shortcuts' tab of Keyboard.prefPane:



I assume that ⌘⇧n as a new plain Finder window was being provided by some other means; Some configuration in that Shortcuts tab (perhaps under services), or a third party app that does a simple "bind these keys to this action", and for some reason that app failed to start, or crashed if you didn't even reboot your Mac.