The problem with our Windows Desktop app is now solved, we apologies for any issues this might have caused you.
To get the Windows Desktop App running again please download the latest version from https://www.wunderlist.com/download/windows7
and install it. After that it should work fine again.
Alternatively you can also simply close the currently installed version completely (also from the systray in the bottom right of your screen), then restart it. This should start the automatic download of the new version.