I think we have nailed down the long startup times for v1.4. If you have existing image caches from v1.3, the app will try to migrate them to a new format for downloading HD card art on starting the app. As such, you may see a blank screen for a longer than usual time.
Please be patient, and it should eventually start up. This will only happen the first time you start Decked Builder after the upgrade.
If you continue to experience problems, please send an email to [email protected] Sorry for any inconvenience caused.