Hi Odiriuss,
In 4.3, we made some changes to Engine headers in order to improve project build times. One of the changes that was made was the removal of the Slate.h include from the Engine.h file. If you have not already done so, you will need to make sure you add this back in.
Quote from the 4.3 release notes:
Gameplay and Framework
- Significant Changes to Engine Headers
- Engine.h has had lots of headers removed to reduce game compile times. We have tried to only remove classes that are unlikely to be used by a game project, but if you see compile errors, you may need to include additional headers.
- Slate.h no longer included by Engine.h, so if you are using Slate, you will need to include it yourself.
If you have made this change and are still seeing build errors related to Slate, please let us know.