The AppBuilder 2.5
update introduced a number of issues in the device simulator that affect the Windows client and the AppBuilder
extension for Visual Studio. Please check the Issues With the Device Simulator After the AppBuilder 2.5 Update forum thread for further information.
I can assure you that we are doing our best to provide a hot fix in the shortest time frame possible. In the meantime, as suggested in the thread, you should be able to use the device simulator in the in-browser client, run your app on a physical device or in the native emulators.
Please, excuse us for the inconvenience these problems are causing in your development process.