How do I fix black UI on Windows?
If you only see a black screen when you open Speedify on Windows this usually means there is an issue with your graphics drivers. Try updating your graphics driver to resolve the issue.
Avast has a great guide for all versions of Windows which you can find here.
If updating your graphics fails to resolve the issue or if you are using an older card, you can run Speedify without GPU acceleration. There's a 'Speedify-NoGPU' shortcut in C:\Program Files (x86)\Speedify, which you can use - just make sure to close all instances of Speedify before running it.