Several users reported that if you upgraded from Winaero Tweaker 0.6.0.0 to Winaero Tweaker 0.6.0.1 in Windows 7 64-bit, the app crashed. The problem appeared only if you did an upgrade and didn't happen with a re-install or a clean install.
I resolved the issue and suggest you re-download the app.
Many thanks to Gaurav Kale for QA, "TropicalGary" for his report and special thanks to PainteR. These people helped to resolve the issue quickly.
Update: I accidentally used files for Windows 10 compiled with .NET 3.5. Replaced them with proper files. If it asks you for installing NET 3.5 in Windows 10, just re-download the app.