Startup error after upgrading to 4.12

After upgrading to 4.12.1 (source build) I get this error message when opening my project:


(It says “Cannot find entry point ?CalculateSlack… of the procedure in the dynamic link library UE4Editor-Core.dll”)

If I press ok the project loads and then the editor starts. Everything seems to work fine (for now).

I tried to rebuild the engine downloading the source from scratch and also deleted all temporary files in my project directory, but the error still appears.