Awh AH! Found the problem!
Because my sources live in a root folder called F:\Sources, the system actually detects the string Source in the path name :
UEBuildTarget.cs CreateModule function:
int SourceIndex = ApplicationOutputPath.IndexOf(Path.DirectorySeparatorChar + “Source”);
So it detects MY root path instead of the project sub folder Source!
So, don’t use the Source string in your root folder!